Hello everyone!
As of a minute ago I have landed the patch removing support for the old fold signature (using ArrayRef<Attribute>). Existing users of the old API will now experience a build breakage and be forced to switch to the new API. See the first PSA for precise instructions of required changes.
With the removal of the old API, the useFoldAPI TableGen option for Dialects has also been removed.
This concludes the migration process as originally laid out in the RFC. Thank you everyone who participated!