The weight of opinion currently seems to be that this site's scope is essentially "stuff humans did in the past". For me personally, the methods used to figure that out I'm not all that picky about. That goes for prehistory as well as historical times. Nit picking about the boundaries of disciplines set up in 20th century liberal arts schools doesn't do anyone any good, particularly when we don't have stacks set up on the other side of most of those boundaries.
So a question about what humans did in prehistory should be decidedly on topic. However, a question about something like early hominid physiology (eg: "Exactly how robust were Australopithecus Robustus' jaws?", or this question) would be an example of a pure anthropology question, and will almost certainly get closed as off-topic.
This seems to be the majority opinion, but it isn't universal. You can see from the other answer it isn't shared by everyone. So if you post a prehistory question, its a pretty good bet that today you will get at least a close vote or two. Whether you get the remaining 3 probably depends on the quality of the question, so you'd better make it a good one. :-)