Wait. This works for Tahoe too? Going to go and mess with it...
[Edit]
It works!! Whoo.
Floating sidebar off, window corner radius set back to 10.
I tried an even smaller corner radius but some apps look weird in the corner if you set it to less than 10. Like, they don't paint all of the way out to the squared-off corner, so you can see some rounding "inside" of the window.
[Edit 2]
Disabling the floating sidebar seems to work in some apps (Finder, Calendar, Reminders, Notes) but not others (Weather, Maps, Find My).