For a little update on where the project is today: we just put out our last update [0] before v1.0, which we're aiming to release later this year. In 1.0, among other things, you'll find many features we've been piloting on our hosted service (Write.as), including newsletters, social media cross-posting, eBook export, etc.
We're also experimenting with new, non-core features, like photo hosting [1] and comments [2]. Our core focus has always been on just writing, which is why many features (including ones afforded by ActivityPub) have been left out. So we're trying to see what's possible when we keep a clean, simple core with optional "rooms" of functionality around it. If you want to keep up with us, you should see more developments around this before the end of the year, too.
[0] https://blog.writefreely.org/version-0-13
[1] https://snap.as