If you deleted the app

Dragged Superposer.app to the Trash without uninstalling first? Steam keeps working. Superposer's small background helper (the login item app.superposer.watcher) notices the app is gone the next time it runs: at login, when Steam starts or quits, or within 30 minutes. It waits two minutes first, so an app update, which swaps the app in place, is never mistaken for a deletion. Then it:

puts Steam back in its normal Mac mode and removes Superposer's tools from Steam. If Steam is open, Steam's per-game settings are cleaned up when you quit Steam (Steam rewrites them as it quits), so quit and reopen Steam once;
removes itself, so nothing of Superposer's keeps running;
records what it did in Superposer's change list and leaves a note, ~/Library/Application Support/Superposer/Superposer was removed - read me.txt.

Your Mac games play as before. Kept: Windows games you installed (Steam may show them as not playable until Superposer is back), their saves and settings, screenshots, the Windows runtimes and Superposer's change record, all in ~/Library/Application Support/Superposer and Steam's steamapps/compatdata.

To use Superposer again: get the app again, drag it to Applications, open it and turn on Steam integration in Setup. It finds your games, saves and runtimes.
To remove everything: install Superposer again and choose Changes to Your Mac › Uninstall Superposer… (it lists everything first and can keep your saves). Or quit Steam and delete ~/Library/Application Support/Superposer (copy your saves out of it first), ~/Library/Caches/Superposer, ~/Library/Logs/Superposer, and the Windows games' folders in steamapps/common with their appmanifest_<appid>.acf files.

If the app was on an external disk that isn't connected, nothing changes until the disk is back or the app is really gone.