I heard about this in F-Droid's latest blog post.
It is based on the Bramble protocol and is compatible with Briar.
It removed mesh-networking features of Briar and added a couple new ones:

• voice messaging
• ML-KEM (post-quantum cryptography)
• disappearing messages

More technical details are shared on their blog.

Sources are on Github https://github.com/zerionproject/Zerion