Allowing megatopics in our multisite cluster causes performance problems so we just restricted this. Note, we do plan to change it so you can increase the PM limit up to 10k.
If you are self-hosted you are free to change these settings as much as you wish. On discourse.org hosting we’ve enforced the 10,000 post count setting globally (as Sam mentioned above), so that cannot be changed.
I.e., the hosting package enforces a 10000-post limit, so it does not matter that @TubaDiva changed the setting (I was kind of wondering why that seemed to have no effect). The discourse.org position is that it places an unacceptable load on their servers (and also that the limit is a feature, not a problem, but my response to that would be that the way I run my forum is none of their business either way; no point in arguing with them about it). How much load I am allowed to put on their cluster is very much mutual business, however.