MSC3440 is currently in a final comment period. We'll need to make a few tweaks to stabilize support for it:

• Use the proper serialization configuration: Change latest_event format in thread bundled relationship #12085
• Support stable identifiers:
  • io.element.relation_senders -> related_by_senders
  • io.element.relation_types -> related_by_rel_types
  • io.element.thread -> m.thread
• Remove experimental configuration flag and unstable identifiers.

We'll probably want to overlap support of stable & unstable identifiers for a couple of releases.

See also: #11753.