MSC2153: Add a default push rule to ignore m.reaction events

proposals/2153-reaction-push-rule.md

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+# Add a default push rule to ignore m.reaction events
+
+Reactions are considered "metadata" that annotates an existing event and thus
+they should not by default trigger notifications. (See
+[MSC1849](https://github.com/matrix-org/matrix-doc/blob/matthew/msc1849/proposals/1849-aggregations.md#event-format)
+for the reaction event format.)
+
+This is especially important for rooms that may be set with a room-specific rule
+to notify, as they will trigger notifications for every reaction and it's tricky
+to clear them as well. See https://github.com/vector-im/riot-web/issues/10208
+for details.
+
+## Proposal
+
+A new default override rule is to be added that ignores reaction events:
+
+```json
+{
+    "rule_id": ".m.rule.reaction",
+    "default": true,
+    "enabled": true,
+    "conditions": [
+        {
+            "kind": "event_match",
+            "key": "type",
+            "pattern": "m.reaction"
+        }
+    ],
+    "actions": [
+        "dont_notify"
+    ]
+}
+```
+
+## Tradeoffs
+
+We could instead allow notifications for reactions in some cases (the current
+state) but then modify each client with complex heuristics to clear them by
+advancing the read receipt. This would be more involved than for regular messages
+because reaction events may target any event in the timeline, so there's no
+guarantee that a new reaction event targets something that's currently displayed
+to the user. By going with the push rule above, client developers won't have to
+deal with this issue.