Re-introduce "Leave out optional keys from /sync"

[callahad]

The changes from #9919 were backed out in #9940 as it broke notifications on Element iOS.

We should re-introduce #9919 as soon as we're able.

Blockers:

• Fix Element iOS iOS notifications are broken with Synapse 1.33.0 element-hq/element-ios#4309
• Wait for a fixed Element iOS release to gain sufficient adoption

[richvdh]

I think we should set a target on doing this work, otherwise it's never going to happen.

@manuroe: I'm going to assume it is realistic that we will be able to do this in three months (ie, by 6th August). Please let us know if you think that is too ambitious.

[manuroe]

The fix will be available in the release we will make in 2 weeks. 95% of people update their app within 1 week.

[deepbluev7]

Also related: element-hq/hydrogen-web#343 matrix-nio/matrix-nio@6097f2c

Although those are merged already at least.

[deepbluev7]

FTR, this is now fixed in iOS. Not released yet though.

matrix-org/matrix-ios-sdk#1100

[tulir]

The release seems to have hit app store 9 days ago, so I guess 95% of people should have updated by now. Can we get this into 1.37? :3