Aggregations: remove not about "future extensions" (#1263)

* Aggregations: remove not about "future extensions"

It is now used by threading.

* Create 1263.clarification

changelogs/client_server/newsfragments/1263.clarification

@@ -0,0 +1 @@
+Fix various typos throughout the specification.

content/client-server-api/_index.md

@@ -1988,11 +1988,6 @@ of times that `key` was used by child events.
 
 The actual aggregation format depends on the `rel_type`.
 
-{{% boxes/note %}}
-This specification does not currently describe any `rel_type`s which require
-aggregation. This functionality forms a framework for future extensions.
-{{% /boxes/note %}}
-
 Aggregations are sometimes automatically included by a server alongside the parent
 event. This is known as a "bundled aggregation" or "bundle" for simplicity. The
 act of doing this is "bundling".