diff --git a/data/api/client-server/capabilities.yaml b/data/api/client-server/capabilities.yaml
index 3379bcd3..6719602a 100644
--- a/data/api/client-server/capabilities.yaml
+++ b/data/api/client-server/capabilities.yaml
@@ -84,6 +84,10 @@ paths:
                         description: |
                           **Deprecated:** Capability to indicate if the user can change their avatar.
                           Refer to `m.profile_fields` for extended profile management.
+
+                          For backwards compatibility, servers that directly or indirectly include the
+                          `avatar_url` profile field in the `m.profile_fields` capability MUST also
+                          set this capability accordingly.
                       m.3pid_changes:
                         $ref: '#/components/schemas/booleanCapability'
                         description: Capability to indicate if the user can change 3PID associations