In reply to Matt.

I'm working on a theme using TwentyTwenty as the foundation and have been doing it on WP 5.4 beta3 (now RC1). So far, I haven't encountered anything of an issue; the exception was that they had the HTML tag as fixed on mobile view (not sure why) in beta2, but I posted it on GitHub (fixed now). Had to make an adjustment to the calendar widget though.