As far as performance issues, even if topics of over 10,000 posts are problematic for their software, I can’t see any reason that a private message thread of over 500 posts would be worse than a public thread of the same size, so I would definitely hope that they’ll be responsive on that point. The reasons they gave for limiting private threads on the linked meta.discourse thread were all about what they consider ideal behavior, which should be for individual forums to determine.
Edit: They launched a new topic on the meta.discourse site about the restriction on 10,000+ topics, although there appears to have been no mention of the much harsher restriction on private messages in that thread.
And yeah, with lack of pagination, I suppose the other issue I’ve noticed is that it makes it much harder to archive a Discourse thread—is there any easy way to do so?
Oh Somehow I had the idea this was referencing some even that had happened on the forum, instead