The context for this is that the Matrix spec allows basically anything
in the device ID. With MSC3861, we're restricting this to strings that
can be represented as scopes.
Whilst this works well for next-gen auth sessions, compatibility/legacy
sessions still can have characters that can't be encoded (mainly spaces)
in them.

To work around that, we added in MAS a behaviour where the device_id is
given as an explicit property of the token introspection response, and
remove it from the scope.
Because we don't expect users to rollout new Synapse and MAS versions in
sync, we needed a way to 'advertise' support for this behaviour: the
easiest way to do that was through an extra header in the introspection
response.

On the longer term, I expect MAS and Synapse to move away from the
introspection endpoint, and instead define a specific API for Synapse ->
MAS communication.

PR on the MAS side:
https://github.com/element-hq/matrix-authentication-service/pull/4067