These terminal commands will disable the floating sidebar (first one) and set the window corner radius back to how it was in Sequoia (second one). To see the effect you have to close and relaunch your app after running the commands. (Log out and log back in to clean up "everything".)
defaults write -g NSSplitViewItemSidebarDefaultsToFloatingAppearance -bool false
defaults write -g NSConvolutionOverride1 -float 10
If you want to set things back to "default", use:
defaults delete -g NSSplitViewItemSidebarDefaultsToFloatingAppearance
defaults delete -g NSConvolutionOverride1