As I commented earlier:
Note that you debate in your example a comment with a comment. While comments should 'improve a post' (semaphore's does this, yours not). If your other comments were of the same kind, then I guess that 'comments are like barncats' is more fitting than anything else? Comments are all volatile. Unless you could point to deleted things more 'substantial for improving the post – and still relevant', then my guess is that an answer to this is probably 'not much' (and shouldn't).
To which you responded in an update to your question:
While comments should 'improve a post' (semaphore's does this, yours not).
What about your comment here? How does it improve my question? Rules for thee but not for me?
While I can relate to your frustration of writing something up that then vanishes by getting deleted, such a fate is really common for comments and not even unheard of for answers. Now that I lifted my comment to which you responded here to an answer box, I increased its chance of survival. Well, I hope so ;). Answers are our first class citizens, comments are not. Improvements for that system may be needed, but most I can think of need to be requested at the system design level at MetaSE.
Those that might be addressed by our policy on comment moderation and how it played out so far for your experience seem to be:
To be fair, indeed I wanted to jot down with that comment a quick response to make you feel heard and less frustrated. While what I wrote is technically also a half-answer – not strictly according to policy, and indeed intended by as such, and still in all too great danger of getting deleted – my 'improvement' part should have been this encouragement:
Unless you could point to deleted things more 'substantial for improving the post – and still relevant'
Meaning: yeah, having your writings deleted does seldom feel very nice, but is a common 'feature' this comment system offers and that under these circumstances – which I am not all too privy to know anything detailed about, they were deleted after all and it's not even easy to figure out which original posts you refer to here – that I recommended to you to make this meta post more accessible and strengthen your point.
So far I haven't been shown here a deleted comment that fulfilled what I would consider 'yeah, should have been left visible, for now'. Do that and I would probably agree and sustain that a comment may have been deleted 'too early' (but see below on why I certainly do not think that there are 'all comments worth preserving for eternity'). I am pretty sure our mods have deleted some comments that should have been up longer, mistakes happen, opinions differ. But in the case as presented I do not agree for the deleted comments to be that important as to constitute a reason of too much concern.
My opinion on that would change if you would point here to some deleted comment on Main like your allusion to a 'Stalin post' — but only if that comment would have been deleted while the big flaw it criticised would be left unfixed. Now that the flaw the comment pointed out is fixed, the comment served its purpose, and may be deleted, or even better should be deleted. Whether by yourself, by mods that read it directly, or anyone flagging it with the canned reason of "no longer needed".
So you did find a very relevant comment, that stayed up for quite a while, and alas also was sadly needed for too long a time. Now that the answer was edited, it may be gone anytime. Why not flag it as 'no longer needed'?
To be clear: I haven't voted either way on your post here. In my opinion – and that's even a little exceptional to 'established by design policy' – some comments are desperately needed to stay below a post. In my opinion those are of the kind: 'adding substantial info' (that should be incorporated into the main post eventually; 'pointing out serious flaws' (of the post they are directed at, and then either taken as base for editing the post or as a reminder of caveat emptor for future readers), etc.
My opinion of 'some comments deserve to stay' quickly diminishes for comments that react to other comments, and that is even evaporating quicker if those sidelined discussions start to devolve away from the original post. These are then maybe interesting, and in my view not forbidden to post, but it is also not guaranteed that they have any right to stay. To the contrary. It is long standing policy that comments are intended for a very specific purpose:
When should I comment?
You should submit a comment if you want to:
- Request clarification from the author;
- Leave constructive criticism that guides the author in improving the post;
- Add relevant but minor or transient information to a post (e.g. a link to a related question, or an alert to the author that the question has been updated).
When shouldn't I comment?
Comments are not recommended for any of the following:
- Suggesting corrections that don't fundamentally change the meaning of the post; instead, make or suggest an edit;
- Answering a question or providing an alternate solution to an existing answer; instead, post an actual answer (or edit to expand an existing one);
- Compliments which do not add new information ("+1, great answer!"); instead,
upvote it and pay it forward;
- Criticisms which do not add anything constructive ("-1, see previous comments you scallywag!"); instead, downvote (and provide or upvote a better answer if appropriate);
- Secondary discussion or debating a controversial point; please use chat instead;
- Discussion of community behavior or site policies; please use meta instead.
And from the beginning of that page:
What are comments?
Comments are temporary "Post-It" notes left on a question or answer.
When one of my comments I think of course relevant but maybe minor gets deleted I am mostly concerned about any OP having read and understood the content. 'Others' are seldom an audience to cater for and upvotes on comments pretty meaningless in most circumstances.
That said, a pretty big problem seems to arise from what I increasingly tend to read out of the chosen title for this question: "self-serving censorship by moderators".
I can't help it but read that
noticed that my comments that are in any way contradicting statements by [mod] get deleted without a trace (not just moved to chat),
as 'I had an argument in comments with a mod, and the mod deleted only my superior comments so that he/his would look better'.
If that's indeed the case, the comments are just the wrong form altogether for the intended purpose.
Maybe I am wrong on how to generalise a formula for this, but unless a deleted comment that reacts to another comment is itself still directly relevant for the original post, I see little reason when such a digressing comment vanishes for the posting user to get too much worked up about it.
I hope that wasn't all too demotivational.
Don't stop in trying to improve the site or the posts on it.
Don't stay upset about a minor comment if that comment doesn't stay up.