If you’d like to take a more active, methodical approach, most plugins include some documentation explaining the new features they add… and all plugins have, at minimum, a “contents” tab, where you can open each shadow tiddler and inspect the code yourself. So you could make a list of any major components added by a plugin (paying special attention to any new widgets/macros/procedures) and then use Advanced Search to see where these features are used in your wiki. If you can’t find any instances in your standard tiddlers or in system tiddlers you’ve created yourself, it’s probably safe to delete the plugin.