Sorry, what are document proxies?
EDIT: Oh, is this it? [0]. I can't say I ever noticed that before (when it still existed)
0: https://osxdaily.com/2014/08/20/open-files-new-app-proxy-ico...
EDIT: Oh, is this it? [0]. I can't say I ever noticed that before (when it still existed)
0: https://osxdaily.com/2014/08/20/open-files-new-app-proxy-ico...
This is the sort of integrated functionality you get when the OS, the application framework and the core applications that come with the OS are all written with each other in mind.
Its under System Settings → Accessibility → Display → Show window title icons