* `m.login.jwt`, which was never specced and has been deprecated
  since Synapse 1.16.0. (`org.matrix.login.jwt` can be used instead.)
* `uk.half-shot.msc2778.login.application_service`, which was
  stabilized as part of the Matrix spec v1.2 release.

Allows specifying a different claim (from the default "sub") to use
when calculating the localpart of the Matrix ID used during the
JWT login.