I'm surprised they're still interested in ActivityPub.
Hot Take: This is good because its easier for people to leave threads, since they can still contact their friends on threads. I do think having most instances block them is also good, so people can have a choice (I personally don't want threads).
I mean, looking at what python3 broke, they are some changes that were well needed. https://docs.python.org/release/3.0.1/whatsnew/3.0.html
I think it would be like xhtml, which broke compatibility with old versions of html, but was (and still is) supported by browsers.