Restore Open Questions section with notes on deferred extensions

Add documentation about webhook notifications and capabilities extensions
that were discussed but deferred to avoid complexity and ecosystem
fragmentation during initial Upload 2.0 rollout.

Author: ewdurbin

peps/pep-0694.rst

@@ -1030,6 +1030,26 @@ you can simply use the placeholder version number ``"0.0.0"``.
 The user that created the session will become the owner of the new project.
 
 
+Open Questions
+==============
+
+Extensions to the Upload 2.0 Protocol
+-------------------------------------
+
+Features such as asynchronous webhook notifications for completion of upload processing
+were discussed during review of this PEP.
+The concept of a capabilities extension for the upload protocol was discussed,
+which would allow implementers to advertise support for optional features
+such as asynchronous notifications or webhooks.
+
+This idea was left open due to the complexity that would arise in designing
+such an extension protocol and ensuring that it did not cause excessive
+fracturing of the ecosystem as Upload 2.0 is rolled out.
+
+Future revisions to the upload protocol should explore such extensions
+as experience is gained operating Upload 2.0.
+
+
 .. rubric:: Footnotes
 
 .. [#fn-action] Obsolete ``:action`` values ``submit``, ``submit_pkg_info``, and ``doc_upload`` are