We want to strip out optional keys by default for /sync, see #9919. This caused issues, so got reverted due to breaking clients #9940, and then got re-introduced again in v1.38.0 #9941 / #10214. It was discovered that this broke Element Android, and so has been disabled again in v1.38.1 #10456 / #10457.

We should reintroduce this change once the Element Android release has propagated sufficiently.

To try and de-risk re-enabling the change we should come up with a plan for how to roll the change out more slowly. The current suggestion is:

• Add a config flag that defaults to the current behaviour (including the key in responses)
• Enable it on matrix.org for a while.
• Flip the flag's default to instead omit the key
• Let that simmer on develop, then RC, then in a release.
• Remove the flag.