Sorry if I missed an announcement, but I assume we should now be using the just-released XD 21 for all panel development? And that any pre-releases are now outdated?
Yes, you can use XD 21 for development. Announcements are in the queues, so should be out soon if not already.
Official XD 21 what’s-new post is now here: XD 21 - plugin panel UI & improved CSS/layout!
And yes, this supersedes any “plugin panel” prerelease builds you were working with previously. You can now submit plugins with panel UI for review & publication to XD users!