@stefan We also saw how Mastodon's quote posts were incompatible with existing quote posting schemas. Mastodon could kinda afford to do this because it didn't have quote posting at all before. We can't break compatibility with existing fediverse replies like that.
So we're sitting on the problem that platforms with interaction controls need to be able to talk to platforms without.
The GTS spec has a section on this. It's very technical, but might show the difficulty: https://docs.gotosocial.org/en/latest/federation/interaction_controls/#optional-but-recommended-behaviors