The specs replace the transports the e2e homeserver advertises with each
shape a migrating deployment may produce and record which token endpoints
each side used: a url-only or combined transport on both sides goes
through the CS API alone, a legacy guest joining a url host uses the
legacy endpoint to publish and the CS API to subscribe, a subscriber falls
back to the JWT service when the homeserver lacks MSC4195, a publisher
gets an error instead, a delayed leave delegated through the CS API fires
on disconnect, and subscribers get tokens for a remote homeserver's SFU
over federation.
The JWT service's app service registration has to cover users of the e2e
Synapse's real server name for the get_token flow, and lk-jwt-service
0.8.0 implements the endpoints.
Johannes Marbach