What are other viable peer-to-peer encrypted communication tools? Is Tox the only alive project in this space?
Briar already has a stable release but I consider it not very viable for most communications because it doesn't yet have the ability to remotely add contacts (you need to scan the peer's QR code from their phone). But they're working on implementing that.
Cwtch isn't out yet, but it is going to implement many of the same features as Briar, with the added perks of having desktop and mobile clients with syncing between the two. It accomplishes this through an untrusted federation of Cwtch servers running over tor to store offline content (when a peer is down) and group chats.
As someone else mentioned Matrix is also a thing. It's not P2P and E2E is still in beta, but it also accomplishes different things compared to Tox et. al. like federating different chat protocols like IRC.
May still be functioning well; did not try it.
There are more recent commits on master they just haven't release in a while.
I think they are waiting for the Tor team to provide an API for v3 hidden services before resuming work on the project.