If you haven’t participated in the 10.21 betas, run the updater and click “get betas” to get it. Release notes here; report bugs here.
If you have a custom scenery pack, please try it on this RC! There have been a few changes to the facade engine to fix cases that were clearly broken in 10.20; please check your scenery to make sure these changes didn’t cause some other problem that I didn’t see in my testing.
In 10.21 RC1, the 4x SSAA lighting looks remarkably better vs. iterations prior. Nice work!