引用自 ツ Seryis ツ：

Currently, when you uninstall a game, it simply deletes the installation directory and removes its shortcuts. This leaves a lot of residual files and folders lying around, and even entries in the registry. This advanced uninstall mode (which would be entirely optional to activate) would perform a more thorough scan of the system to locate all residual files related to that game and remove them, including registry entries. It ends up being tiresome to have to resort to Revo Uninstaller to uninstall games effectively.

引用自 Ben Lubar：

Developers can tell Steam to remove additional folders and registry entries:

https://partner.steamgames.com/doc/sdk/installscripts

However, most opt to leave your save files and settings in place in case you reinstall the game. (And because they're not terribly big files to begin with.)