MSC4210: Remove legacy mentions (#4210)

* Remove legacy mentions

Signed-off-by: Tulir Asokan <tulir@maunium.net>

* Add another potential issue

Signed-off-by: Tulir Asokan <tulir@maunium.net>

* Add new room version to alternatives

Signed-off-by: Tulir Asokan <tulir@maunium.net>

* Specify that including `m.mentions` in all events is still necessary

Signed-off-by: Tulir Asokan <tulir@maunium.net>

* Add links to current spec

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Signed-off-by: Tulir Asokan <tulir@maunium.net>

proposals/4210-remove-legacy-mentions.md

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+# MSC4210: Remove legacy mentions
+Matrix v1.7 introduced [intentional mentions], where events list users they
+mention explicitly, instead of the recipients inferring mentions from the raw
+message text. For backwards-compatibility reasons, messages without the new
+`m.mentions` field still use the old plaintext matching for mentions.
+
+[intentional mentions]: https://spec.matrix.org/v1.15/client-server-api/#user-and-room-mentions
+
+Plaintext matching means it's very difficult for automated tools to tell which
+users are mentioned in a message. This means that it's easy to spam mentions by
+simply not using intentional mentions.
+
+If intentional mentions are mandatory, automated tools could easily ban users
+who send more than X mentions in a single message. There could even be a new
+push rule condition to allow checking the number of mentioned users and skip
+notifying entirely.
+
+## Proposal
+Support for legacy mentions is dropped. Specifically, the following deprecated
+standard push rules are removed entirely:
+
+* [`.m.rule.contains_display_name`](https://spec.matrix.org/v1.15/client-server-api/#_m_rule_contains_display_name)
+* [`.m.rule.contains_user_name`](https://spec.matrix.org/v1.15/client-server-api/#_m_rule_contains_user_name)
+* [`.m.rule.roomnotif`](https://spec.matrix.org/v1.15/client-server-api/#_m_rule_roomnotif)
+
+Additionally, the `contains_display_name` [push rule condition] is deprecated.
+
+[push rule condition]: https://spec.matrix.org/v1.15/client-server-api/#conditions-1
+
+Including an empty `m.mentions` key is still required for clients that are
+aware of intentional mentions, as omitting it would cause current clients to
+assume messages are not using intentional mentions.
+
+## Potential issues
+Users using old clients (which don't send intentional mentions) will no longer
+be able to mention users on up-to-date clients/servers.
+
+Users using old clients (which don't support the new push rule conditions) will
+also no longer be notified for mentions in case the client depends on the push
+rules served by the server.
+
+## Alternatives
+The removal could be done in a new room version, such as when switching to
+extensible events, as suggested by [MSC3952]. However, such a migration will
+likely take much longer than clients implementing intentional mentions.
+Additionally, the room upgrade UX is still an open issue, which means many
+rooms simply don't upgrade. Therefore, making a slightly breaking change to
+existing room versions seems like the better option.
+
+[MSC3952]: https://github.com/matrix-org/matrix-spec-proposals/pull/3952
+
+## Security considerations
+This proposal doesn't add any features, so there are no new security
+considerations.
+
+## Unstable prefix
+Not applicable, this proposal only removes features.
+
+## Dependencies
+None.