MSC4210: Remove legacy mentions

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.
+
+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`
+* `.m.rule.contains_user_name`
+* `.m.rule.roomnotif`
+
+Additionally, the `contains_display_name` push rule condition is deprecated.
+
+## Potential issues
+Users using old clients will no longer be able to mention users on up-to-date
+clients/servers.
+
+## Alternatives
+
+## 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.