MSC3989: Redact origin property on events

proposals/3989-redact-origin-field.md

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+# MSC3989: Redact `origin` field on events
+
+The [current redaction algorithm](https://spec.matrix.org/v1.6/rooms/v10/#redactions) *keeps* the
+top-level `origin` field on events during redaction, however the only use of it as of writing is
+a malformed example of event format. The field has no significant meaning in modern room versions.
+
+Note: some other fields are additionally useless in modern room versions, however they are already
+adapted by [MSC2176](https://github.com/matrix-org/matrix-spec-proposals/pull/2176).
+
+This proposal's scope includes *removing* other fields which are useless in modern room versions,
+however makes no effort or consideration for *adding* protected fields to the redaction algorithm.
+Given MSC2176, the only useless field appears to be `origin`, however the author kindly asks that
+reviewers point out other examples as they arise.
+
+## Proposal
+
+In a future room version, the `origin` field 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)