Remove unstable_features endpoint

Author: babolivier

proposals/2320-identity-versions.md

@@ -1,14 +1,11 @@
 # Versions information for identity servers
 
 The client-server API currently specifies a `/versions` endpoint that allows
-clients to know what version of that API and unstable features are implemented
-by the server. This allows MSC to be implemented and supported by both servers
-and clients before they're merged and incorporated into the stable API.
-
-This is also a feature we will need on the identity server API, because
-clients talk to them and need to be aware of unstables features (such as
-[MSC2265](https://github.com/matrix-org/matrix-doc/pull/2265)) when they're
-implemented by the identity server it's using (if there is one).
+clients to know what version of that API are implemented by the server.
+Identity servers could benefit from that endpoint as both homeservers and
+clients interact with them, and therefore could know which features they can
+expect a given identity server to implement by looking at the versions of the
+API it claims to support.
 
 ## Proposal
 
@@ -29,26 +26,10 @@ this identity server supports. Its response uses the following format:
 }
 ```
 
-### `GET /_matrix/identity/unstable_features`
-
-This endpoint serves information about the unstable features, i.e. features
-specified in a MSC or an unstable version of the Matrix specification but not in
-a stable one, supported by the server. Its response uses the following format:
-
-```json
-{
-    "unstable_features": {
-        "org.matrix.casefold_email_addresses": true
-    }
-}
-```
-
 ## Alternative solutions
 
 Another solution which was considered was using the status check endpoint ([`GET
 /_matrix/api/v1`](https://matrix.org/docs/spec/identity_service/latest#get-matrix-identity-api-v1))
 to serve this information. This solution was discarded because it's using a
 versioned endpoint, which doesn't make sense to advertise the supported versions
-of the API to use, and this endpoint was serving both the supported versions and
-the supported unstable features, whereas it makes more sense to have each of
-these pieces of information served on a different endpoint.
+of the API to use.