the main bit of auth which was left unimplemented on matrix-workers was the critical logic which authorizes traffic over federation: https://spec.matrix.org/latest/server-server-api/#authorizat...

Auth for clients is also specified in the spec - there is some scope for homeservers to freestyle, but nowadays they have to implement OIDC: https://spec.matrix.org/latest/client-server-api/#client-aut...