Apple - How can I uninstall Safari Technology Preview?

I used AppCleaner.app to make sure I don't have any leftovers. And it turned out there were a lot, namely crash reports, plists, diagnostics, cookies and more.

Just make sure no Safari files are included in the list of files about to be deleted. In my case there were no such files, it was either plugin related crash reports (e.g. VLC plugin) or other files clearly related to Safari Technology Preview.

Exactly. To delete an Application that is not sandboxed (not downloaded from App Store) you have to go in /Applications/ and delete the Application File.