Had a hard time finding any recent information either, but with the recent release of 1.20 I've noticed that an Activitypub endpoint got added in the API and that permissions for it can be added to tokens. Seems very rudimentary so far though.
The ActivityPub extension ForgeFed for federation between different servers is not finished yet. AFAIK Forgejo contributors work closely together with other ForgeFed devs to bring it forward.