The deleted answer and its quality
There is this well researched, amply referenced, and dotted with examples and quotes, truthful answer that addresses the question as asked, in much detail.
It does this with the most relevant source in book form plus explanations, scientific as well as political and historical.
We see nine upvotes on it, and that it was accepted by OP.
We also see it deleted.
The deletion process timeline
One delete vote from long ago that never aged away from a user that himself edited the answer much to my dislike into a shape that attracted via timeline the majority of downvotes.
Two more recent delete-votes from users who in the past repeatedly professed to be not into reading long answers.
None of the delete votes seem to conform to the criteria set forth in the documentation 'when should a user cast deletion votes'.
Deplatforming attacks on the question
Via timeline for the question we see also that the question itself is valid enough to be open now. But as well that it was under a deplatforming attack from users who do not want the topic existing at all. One of those previous deplatforming close votes coming from one of the answer delete voters.
That leaves a direct answer hidden from view and an existing answer in place that really isn't a complete and self-reliant answer at all, but merely a comment and expansion to the deleted answer.
Quality of deleted answer as per reactions in comments and chat
One comment below the deleted answer is:
It's unfortunate that your answer starts in such a provocative way. Had you replaced the first few paragraphs with something more neutral, to the effect of "vaccines weren't as safe as they are now" you'd never have had such a high number of downvotes.
That is indeed unfortunate. If current day activists and propagandists want fallaciously to 'appeal to consequences' now on an upvoted and accepted answer… that's telling! But not in a good way. It's not 'healthy' for the site.
Another comment shows the utmost of ignorance in refusal to read through the actual answer. It says:
this is a fringe theory that is not accepted by the scientific mainstream. As a result, this is heavily downvoted.
"Fringe theory"? When quoting medical authorities like the CDC, or mainstream historians on the very subject matter like Stanley Williamson: "The Vaccination Controversy. The rise, reign and fall of compulsory vaccination for smallpox", Liverpool University Press? That's really ridiculous and a clear sign of not bothering to read but jumping down into the comment section to vent unsubstantiated opinions.
And the quite happy first comment on it was:
Excellent quote from Durbach, thanks. The low quality of free vaccinations is an eye-opener.
Sidenote for those accusations about any 'agendas'; the still standing and upvoted but isolated half-answer in response and as comment to the deleted one says about the deleted one:
And another comment is clearly incomprehensible as it is one major point in the argument presented in the answer:
I can't help it, but for me that picture of the "horrible side effects of smallpox vaccination" is virtually indistinguishable from a picture of a smallpox victim...
Well, yeah, exactly the point. Ordinary people do not see much difference – and then conclude that smallpox vaccinations are maybe not as safe for them as propagandists always claim.
Perhaps this really accomplishes something. For lay people this is, and now image 19th century people delivering their child to the station, get a scar under dreadful conditions then return home being sick from whatever, but even worse: the younger sibling looking like this 2 weeks later. What would your own conclusion be? And you see from the colour that this is a more recent picture. (But thx for headsup, forgot to link outside alt-tag)
To which the detractor simply doubles down with:
I wish I could give more downvotes.
For explaining historical facts, medical facts, psychological and sociological explanations, all referenced? This looks like someone in some denial.
Yet another comment correctly concludes:
The answer is well researched and documented. This is what should be taken into account while voting for answers. Whether someone perceives the answer as being pro or against vaccines is neither here not there.
And yet that's all we see from a certain subset of users here.
The fallacy of judging a historical answer by 'appealing to consequences' for contemporary present day politics agendas is even made quite explicit:
But when taking into account the risk of contagion when not vaccinated, as well as the effect of people refusing vaccination on principle when we as human society are trying to erradicate a disease (and hence the need for vaccination in future generations), summarizing as (boldface) "vaccinations are dangerous" is misleading at best.
Now that's not what I call sine ira et studio.
Dealing with criticism, whether valid or invalid, and the H:SE catch-22 of resolving disputed content
The answer was criticised heavily. With debating comments challenging any unreferenced claims they didn't like as 'untrue', 'misleading'.
The proper way of dealing with such allegations is a) correcting the content, b) removing the wrong content if allegations are true or c) proving them untrue.
With proof and evidence in the form of quotes and references from reputable sources.
Which I did. Everything written and challenged is backed up. We see the data, we see the experts, we see the sources.
What else can an historical answer provide than going ad fontes and contextualising this with other expert opinions? The detractors below that deleted answer don't have a leg to stand on, when they apparently want to try to answer 'why was it discontinued' with 'people were all stupid back then', or 'evil did triumph once more'. This kind of moralising judgement is the very opposite of what historical research, or really any kind of research should do.
The style of discussion in comments, chat and on meta
Further, I'd like to voice my concerns here about an active subset of the community that places their personal feelings and perceived moral obligations before historical research and results.
This is a phenomenon now seen quite a bit too often already. It is quite unnerving to so often see cries of 'biased' and 'untrue' in comments below posts from users who cannot properly backup their most often than not unsourced opinions.
From the post under discussion here we see at least 5 users not reading what is written. Consequently misunderstanding, some quite wilfully, what the answer says.
Jumping to the comments without reading a post is quite senseless. Voting on a post without reading it is senseless. It needs to stop. The built-in technical limitation for a post is 30000 characters. Anyone who wants to complain about that needs to vent on MetaSE and not below posts that use this provision. Preferably on the post were Shog9 requests this limit even to be raised?
Keeping it deleted?
The above means that I see no reason for that post to remain deleted. Especially not when compared to all those other posts, like those without any necessary references of more often than not more questionable quality.
Simply a net negative score surely cannot be the only reason to delete such an answer?
Some contention arose from sheer length, which I consider a wholly invalid complaint altogether, and always. On that topic specifically many benign sources like Wikipedia also presents any reader coming along with unpleasant pictures. Disputed facts are properly referenced. Among them the much controversy provoking 'magic word' "dangerous", which coincidently appears all the time in the quoted quality references, whether from relevant primary sources at the time or later historical depictions, or *current medical expert opinion. Whether it is the best word here or not will remain a matter of personal opinion of readers. And one magic word should not be sufficient reason to delete an answer.
Given that most of the downvotes obviously – and in part also documented by comments – came from campaigning deplatformers, and that one delete vote came from a once agreeing editor that never aged away and the next two were from knee-jerk no-readers, I'd like to suggest to the community to undelete a well referenced answer that directly answers the question with some of the best available sources, which was upvoted nine times and accepted by the asker.
Answer under discussion: Why was compulsory vaccination abandoned in the UK?
Frozen chat room for it: https://chat.stackexchange.com/rooms/99373/discussion-on-answer-by-langlangc-why-was-compulsory-vaccination-abandoned-in-th
After time for contemplation, deliberation, and only two meagre voices on this here in answers: undelete now!
Taken together, this looks pretty much like a plain wrongful deletion, against policy, not meeting required criteria as outlined in for example our help-center or meta.stackexchange.com/q/5221
Even after inquiry here so far no valid criteria warranting deletion came forward. Instead we saw – still after editing by mod for politeness and operational focus – personalised attacks on 'style' and 'being no asset', "verbal diarrhea", "very little is" (compare with who's posting the insults here).
"Too long" isn't one, the deleted answer is not campaigning anything (unlike commentators and del-voters) and it does answer the question, with facts, referencess, sources, quotes. All of them of high quality, again contra accusations of any 'agenda'.
It should be undeleted at once, because the deletion voters and process itself failed the policy of meeting the required criteria to cast a deserved delete vote.
As a reminder on the established network policy for delete votes and sheer post length:
Links to external resources are encouraged, but please add context around the link so your fellow users will have some idea what it is and why it’s there. Always quote the most relevant part of an important link, in case the external resource is unreachable or goes permanently offline.
From there, special attention needs to be paid to "What are the criteria for deletion?":
Answers that are wrong or that dispense poor advice should be downvoted, not deleted.
And the deleted one isn't 'wrong', but it is answering the actual question, not giving any advice – at all.
Now remember it: the deleted one was the accepted answer:
If I flag my post with a request to delete it, what will happen?
If you cannot delete your answer, then it must have been accepted by the question author. One of the main points of these sites is to provide help to others searching for answers; deleting your answer that the author has indicated as helpful to them detracts from maintaining such a knowledge base.
Moderators will not delete your post for the above reasons.
Since the existing answers up to this date here on meta failed to provide a meaningful explanation or give valid reasons for deleting that post, but instead proved nothing but the reason given so far to be a primitive 'no-likey', I hereby request the undeletion of a useful post.