Ehhhh, Not a fan of Matrix but I haven't investigated it a ton. What I'm looking for probably doesn't exist.

Central Login Server hosted by Foundation of Chat Program and servers authenticate via OAuth2.

Central Login Server keeps track of which Servers identity is in and their endpoints so people don't lose track of servers.

Central Mobile Notification Service because mobile app notification is so important AND SO PAINFUL TO RUN.

Federation NOT required.

fwiw Matrix does do this, although making the OIDC IdP track the right target homeserver might need a bit of config.