[SDK] matrix-android-sdk2 uses an unspecced `original_event` field from the `/relations` response

[clokep]

Description

The /relations endpoint does not return an original_event field, yet the matrix-android-sdk2 uses it.

We would like to remove support for this from Synapse in < 6 months. See matrix-org/synapse#12930 for more context.

[clokep]

From a quick look of consumers, it seems that this is actually used, but I am not 100% certain:

• https://github.com/matrix-org/matrix-android-sdk2/search?q=originalEvent
• https://github.com/vector-im/element-android/search?q=originalEvent

[bmarty]

Yes, this is used. We can try to un-use it, I will check if it's possible to do that fast.