With the old IDE, when I close the only sketch I have open, the IDE stays open - that is, I still have an Arduino menu bar at the top.
With the BETA version, when I close all the sketches, the IDE closes completely, and I have to restart it to open another sketch.
This may be by design, however, I prefer the former option, that leaves the IDE open until I exit from the program. Often, I end up referencing many sketches at a time and then start closing them as I no longer need the reference, if I accidentally close them all, then I have to restart the IDE. Also, anecdotally, I believe this is how many other macOS apps behave - they stay open until explicitly closed.
Just a thought!
I am using macOS Big Sur, and BETA Version: 2.0.0-beta.7