Note by @fastfinge

@IceWolf @foxbutt Nah, it depends on how your implementation is configured. Some server owners turn off backfilling because they want to save disc space and don't care about search. And some server owners configure things so that there server will only show your server a certain subset of posts from a user, rather than all of them when it asks. And then authorized fetch and how it interacts with blocking and post privacy adds another layer of complexity.

And, of course, none of this stuff is (or can) be enforced by any kind of technical server. Someone could easily write/patch an "evil mastodon" to suck up as many posts as it can, while fooling the other server into thinking the requests are legit. Kind of like how some torrent clients are written to upload as little as possible.