Add backwards-compatibility notes.

proposals/2414-optional-content-reporting-reason.md

@@ -8,6 +8,12 @@ This MSC proposes that the `required` flag for this parameter be removed, as
 well as the "may be blank" clause in the description.
 
 Note that the kicking and banning endpoints already have optional `reason`
-parameters. The other endpoints mentioned in #2367 will likely also add
+parameters. The other endpoints mentioned in [#2367](https://github.com/matrix-org/matrix-doc/pull/2367) will also add
 optional `reason` parameters, so it seems that it would be more more consistent
-with the rest of the spec to make this optional as well.
+with the rest of the spec to make this optional as well.
+
+## Backwards Compatibility
+
+Since servers currently expect this field to be sent by all clients, making
+this optional is a breaking change. Clients should send query the version of
+the homeserver to determine whether they can safely omit this field.