MSC3786: Add a default push rule to ignore m.room.server_acl events

proposals/3786-acl-notifs.md

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+# MSC3786: Add a default push rule to ignore `m.room.server_acl` events
+
+`m.room.server_acl` events allow for expressing which servers can participate in
+a room. Room server ACLs aren't something the user should have to worry about,
+so these events should not trigger notifications; however right now they *do*
+trigger notifications, if the user has the room rule set to `notify` (`All
+messages` in Element).
+
+Additionally, these events are often sent quite frequently during spam attacks,
+which causes the users to be overwhelmed by
+notifications. (See <https://github.com/vector-im/element-web/issues/20788>.)
+
+Due to these problems, this MSC proposes a new push rule to ignore these events.
+The new push rule is analogous to `.m.rule.member_event` (see <https://spec.matrix.org/v1.2/client-server-api/#default-override-rules>), or `.m.rule.reaction`
+(from [MSC2153](https://github.com/matrix-org/matrix-spec-proposals/pull/2153)
+or [MSC2677](https://github.com/matrix-org/matrix-spec-proposals/pull/2677)).
+
+## Proposal
+
+A new [default override
+rule](https://spec.matrix.org/v1.2/client-server-api/#default-override-rules) is
+to be added that ignores `m.room.server_acl` events:
+
+```json
+{
+    "rule_id": ".m.rule.room.server_acl",
+    "default": true,
+    "enabled": true,
+    "conditions": [
+        {
+            "kind": "event_match",
+            "key": "type",
+            "pattern": "m.room.server_acl"
+        },
+        {
+            "kind": "event_match",
+            "key": "state_key",
+            "pattern": ""
+        }
+    ],
+    "actions": []
+}
+```
+
+This new push rule is inserted immediately after `.m.rule.tombstone`.
+
+## Unstable prefix
+
+During development, `.org.matrix.msc3786.rule.room.server_acl` is to be used
+instead of `.m.rule.room.server_acl`.