Remove unstable MSC2858 APIs

[callahad]

Followup to #9627 for APIs defined in matrix-org/matrix-spec-proposals#2858

We're aiming to implement this no sooner than the first release in August. By the calendar, that should be Synapse 1.38 1.40. Thus, we can implement any time after 1.37 1.39 has branched.

Blockers:

• Synapse 1.37 1.39 has been branched
• Switch to stable endpoint/fields for MSC2858 element-hq/element-ios#4362

Tasks:

• Notify client teams of impending change and get sign-off
  • Element iOS
  • Element Android
  • Element Web
• Disable experimental flag on matrix.org
• Monitor for a few days
• Remove code from develop

[richvdh]

The first release in August is going to be 1.40, for the record.

[richvdh]

MSC2858 support is gated by an experimental flag, so the first step here is probably to turn that off on matrix.org.

[callahad]

@richvdh notes that we can check if anyone is calling the _unstable endpoint.

[callahad]

@richvdh to do that, and @reivilibre to observer (welcome (back) to the team!)

[reivilibre]

I went through the haproxy logs (thanks for your help Rich!) on matrix.org and found that we have roughly between 20 and 100 hits to this unstable API per hour.

I took a sample of one hour on one haproxy and found that all 47 requests were from iPhones.

There is already an issue to switch to the stable API, which I have commented on: element-hq/element-ios#4362 .

[reivilibre]

N.B. just for the sake of it, I found element-hq/element-android#3442 — Android has already changed over

& element-hq/element-web#17521 — web too

[richvdh]

The element iOS team expect to have an update in the app store in two weeks, and recommend waiting another week after that, for 95% penetration of the new version.

So, we should be able to revisit this w/c 23rd August.