Can you add Tags to pre-installed apps on macOS Catalina? I'm trying to add Finder Tags to some of my apps in Catalina, but I notice it doesn't seem to let me add tags to pre-installed apps (Messages, News, etc.). Is this still possible or has this functionality been removed due to "ready-only" system partition on Catalina?
No you can’t add tags to the pre-installed apps.  However you can right click the app in Finder and choose the option to "Make Alias" and you can add a tag to the alias file.