Unclear what /user/devices should return when user or keys are not found

https://matrix.org/docs/spec/server_server/r0.1.4#get-matrix-federation-v1-user-devices-userid

It looks like Synapse always returns 200, but shouldn't 404 make more sense?

Start with the linked Matrix server-server specification section for GET /_matrix/federation/v1/user/devices/{userId}. Compare its stated behavior with the reported Synapse response when the user or keys are missing. Done means the expected status and response behavior are decided and the specification is updated accordingly.

Written by the indexing model from the issue text.