I generally run http://freemacsoft.net/appcleaner/ when I want to trash an app, so I was pleasantly surprised that hazel found some items that maybe I hadn’t run appcleaner on.
When I turned on the feature in Hazel, it picked up cruft from 5 apps that had been previously deleted. Very cool feature and am glad that I turned it on.
If the app is still in the trash, then Hazel (if it has Full Disk Access) can see the app in the trash, and then find the related files and offer to delete them.
If you have emptied the trash, obviously Hazel won’t be able to help because it won’t know what apps to clean up after.
Also handy: if you reinstall the app, and Hazel realizes that the app is back and Hazel had “App Swept” the related files to the trash, it will prompt you and ask if you want those files back (you might or might not, depending).
I’ve had this Hazel feature activated for quite a few years. It is very helpful.
It’s important to review the list of files that Hazel proposes deleting to be sure they should be removed. In some cases I’ve found that it wants to delete data files that I want to keep.