How should an appservice opt-out of providing an endpoint?

Nobody has claimed this yet.

It's often the case that an appservice does not provide all of the functionality that is in https://spec.matrix.org/v1.10/application-service-api/#querying. For example, very few appservices actually implement the `/_matrix/app/v1/users/{userId}` endpoint. If you're just interested in consuming events, you really don't need the others.

However, there isn't a spec-defined way to opt-out of providing these endpoints. Most developers opt to just let them 404 or 5XX or something to that effect, and hope the homeserver gracefully handles the error. I think we should do better here, and find a way to describe the endpoint as unimplemented.

Given that 404 is already taken as a valid response from some of these endpoints, one idea is just to respond with 501 (Not Implemented) for any valid endpoint that the appservice does not support. This would cause the homeserver to exempt that appservice for the duration the homeserver is up.

Anyway, if this is a reasonable sounding idea this should probably become a spec.

Start with the Matrix Application Service API section on querying, especially the /_matrix/app/v1/users/{userId} endpoint, and compare its defined 404 behavior with the proposed 501 response. Done would require an agreed spec-defined opt-out mechanism and corresponding specification text; the issue names no repository files or tests.

Written by the indexing model from the issue text.