FYI: Stephan Casas, who was the person who originally discovered the flag to disable the new style text cursor, helped me to find a flag that will disable new input source switcher popup without the unwanted side effect of the text cursor not showing up in some circumstances.
This won't disable the CapsLock indicator, however, there might end up being another flag for that (see the discussion in the link below).
To disable just the new style language/input switcher:
First, if you had used the previous flag to disable the new style text cursor completely, you should probably delete it as described above using:
sudo defaults delete /Library/Preferences/FeatureFlags/Domain/UIKit.plist redesigned_text_cursor
You'll probably need to restart your system to re-enable the new style text cursor.
After that, disabling the inline input switcher pop up is as simple as:
defaults write kCFPreferencesAnyApplication TSMLanguageIndicatorEnabled 0
That makes the system start using the old center screen HUD style input method switcher immediately, without needing to restart your system.
Any open applications will still show a popup when the input method is changed. However, once an app is restarted, it will no longer show an indicator when it's changed.
That's more than enough for me. Hopefully Apple won't change it. :)
Again, this won't disable the CapsLock indicator.
See here for further discussion: https://gist.github.com/stephancasas/236f543b0f9f6509f5fe5878de01e38a?permalink_comment_id=4748936#gistcomment-4748936