Original proposal: https://github.com/matrix-org/matrix-doc/pull/1659
Implementation proofs (some traversing of the PR tree may be required to get all of them):
* https://github.com/matrix-org/synapse/pull/4483
* https://github.com/matrix-org/synapse/pull/4499
This doesn't intentionally change anything from the proposal.
**Implementation details**:
The simple part of this is the introduction of a rooms/v3.html document. The somewhat unclear part is the stuff done to the s2s definitions. This pulls `unsigned_pdu` out to `unsigned_pdu_base` (all fields except `event_id`) where it can be reused in `pdu` and `pdu_v3` (for rooms v3). These definitions are further moved into the room version specifications where they can highlight the exact schemas in detail.
Version 1 has been updated to include the pre-existing event format, however the core principles of the room have not been changed. The same applies to room version 2. Room versions have immutable core principles once in the spec, otherwise these format changes would land in a pre-existing version.
The client-server API event formats will need updating, however that is being punted to a different commit to try and keep these changes reviewable.
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