You may want to check out https://f-droid.org/en/packages/com.cheogram.android/ for replies, custom emoji, and other fun stuff. e2ee options for reactions are specced and being worked on, just takes time to get things implemented with a small group of devs 🙂

some of us never left

I think used is an overstatement. They had a half-built gateway, but it wasn’t part of the main product anyone used (I used it, but their main apps did not)

Unfortunately this is largely mythological. There were very few efforts by major players towards truly adopting any standard or even open API.

Nothing nearly as big but FOSSY is organized on similar principles.