MSC3932: Extensible events room version push rule feature flag

proposals/3932-extensible-events-room-ver-push-rule-condition.md

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+# MSC3932: Extensible events room version push rule feature flag
+
+As defined by [MSC3931](https://github.com/matrix-org/matrix-spec-proposals/pull/3931), push rules
+can be disabled or enabled based on what the room version itself supports feature-wise. This MSC
+introduces a feature flag for [MSC1767](https://github.com/matrix-org/matrix-spec-proposals/pull/1767),
+making it possible to restrict extensible events to an appropriate room version through notifications.
+
+A theory with this approach is that if push rules (and therefore notifications) are denied for extensible
+events, they are even less likely to be sent in inappropriate room versions.
+
+## Proposal
+
+For room versions supporting extensible events, they declare support for an `m.extensible_events`
+feature flag.
+
+Push rules which don't specify a `room_version_supports` condition are assumed to *not* support
+extensible events and are therefore expected to be treated as disabled when a room version *does*
+support extensible events. This is to ensure that legacy rules do not trigger in new room versions,
+similar to how this MSC aims to only enable certain push rules in applicable room versions.
+
+The above rule does *not* apply to the following push rule IDs:
+* `.m.rule.master`
+* `.m.rule.roomnotif`
+* `.m.rule.contains_user_name`
+
+Note that this MSC intentionally considers the lack of the condition rather than the lack of feature
+flag check on the push rule: this is intentional as the timeline for extensible events appears to
+show that this "push rules feature flags" system will land at the same time as extensible events,
+causing all future MSCs to consider extensible events too.
+
+Any room version which supports MSC1767's described room version is required to list this new feature
+flag.
+
+## Potential issues
+
+There is complexity in understanding how to apply the feature flag's condition to existing push rules,
+however through implementation effort it will become clearer.
+
+## Alternatives
+
+None discovered.
+
+## Security considerations
+
+No new considerations apply.
+
+## Unstable prefix
+
+While this MSC is not considered stable, implementations should use `org.matrix.msc3932.*` in place of
+`m.*` throughout this proposal.