MSC3989: Redact `origin` property on events (#3989)

* Redact `origin` field on events

* Provide context and remove hunt for more useless fields

* spelling: actually

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Slight wording alterations

* Property

---------

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

proposals/3989-redact-origin-field.md

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+# MSC3989: Redact `origin` property on events
+
+The [current redaction algorithm](https://spec.matrix.org/v1.6/rooms/v10/#redactions) *keeps* the
+top-level `origin` property on events during redaction, however, as of this writing, the only use within the
+spec of `origin` as a top-level event property is a malformed example of event format. The property has no
+significant meaning in modern room versions.
+
+Within the ecosystem, it's clear that we'd [prefer the property to disappear](https://github.com/matrix-org/matrix-spec/issues/374),
+and have [tried to do so](https://github.com/matrix-org/matrix-spec/pull/998) in the past. The
+malformed examples are even [known to us](https://github.com/matrix-org/matrix-spec/issues/1480).
+
+What's not clear, and mentioned in [a comment](https://github.com/matrix-org/matrix-spec/issues/1480#issuecomment-1495183789),
+is whether the `origin` property is *actually* used. There do not appear to be any auth rules or similar
+which would use the property, however it'd hardly be the first time that the spec was wrong about an
+ancient room version like v1. What is clear is that Synapse, where this question would be asked,
+wants to [drop support](https://github.com/matrix-org/synapse/issues/3816) for the property and has
+taken [steps](https://github.com/matrix-org/synapse/pull/8324) towards that mission by fixing bugs
+in the area. In a [quick audit](https://github.com/matrix-org/matrix-spec-proposals/pull/3989#issuecomment-1497659507)
+of the Synapse codebase during implementation of this MSC, the `origin` property appears unused.
+
+Given the above context, this proposal removes the `origin` property from the [redaction algorithm](https://spec.matrix.org/v1.7/rooms/v10/#redactions)
+in a future room version, leaving it as-is for existing versions (not that an MSC can change the behaviour
+of an already-stable room version anyways).
+
+Some other properties are additionally useless in modern room versions, however they are already adapted
+by [MSC2176](https://github.com/matrix-org/matrix-spec-proposals/pull/2176).
+
+## Proposal
+
+In a future room version, the `origin` property is *removed* from the list of *event* keys which are
+kept during redaction. Note that this requires a new room version because changing the redaction
+algorithm changes how [event IDs](https://spec.matrix.org/v1.6/rooms/v10/#event-ids) are calculated,
+as they are [reference hashes](https://spec.matrix.org/v1.6/server-server-api/#calculating-the-reference-hash-for-an-event)
+which redact the event during calculation.
+
+## Potential issues
+
+No major concerns.
+
+## Alternatives
+
+No significant alternatives.
+
+## Security considerations
+
+No major concerns.
+
+## Unstable prefix
+
+While this MSC is not considered stable, implementations should use `org.matrix.msc3989` as the room
+version identifier, using v10 as a base.
+
+## Dependencies
+
+No blocking dependencies.
+
+This MSC would partner well with the following MSCs, however:
+* [MSC2176](https://github.com/matrix-org/matrix-spec-proposals/pull/2176)
+* [MSC2175](https://github.com/matrix-org/matrix-spec-proposals/pull/2175)
+* [MSC2174](https://github.com/matrix-org/matrix-spec-proposals/pull/2174)
+* [MSC3821](https://github.com/matrix-org/matrix-spec-proposals/pull/3821)