Note: Please do not manually delete files from the /workshop/content/ directory. Steam will still expect them to be there, and will reinstall those mods when verifying game files. To uninstall a mod, first make sure you are unsubscribed to it.
When you manually subscribe to a Steam Workshop mod, or join a modded server, Steam will keep those files installed for a certain amount of time even if you unsubscribe. This is (presumably) so that if you were to resubscribe in the future, those files do not need to be redownloaded.
Only subscribed files are loaded during startup, so there is no performance penalty to having non-subscribed files installed. Mods can also be manually disabled from your in-game mods list.
At the time of writing (2023-06-02), Steam does not delete a mod's root folder when uninstalling content. This may make it appear as if more content is installed than there actually is.