Reference
Steam shader cache
What the Steam and GPU shader cache is, where it lives on Windows and Linux, whether it is safe to delete, and why it keeps growing after you uninstall a game.
Proton prefix (compatdata)
What Steam keeps in compatdata, why each folder is gigabytes, and why it is the one leftover you must not delete without looking first.
libraryfolders.vdf
The file that tells Steam where your libraries are and which games live in each — how it is structured, how it breaks, and why editing it by hand is risky.
appmanifest_*.acf
The small file that makes Steam consider a game installed — what is in it, what StateFlags means, and why deleting one frees no space at all.
Steam Workshop content
Where subscribed Workshop items are stored, why they survive uninstalling the game, and the difference between deleting the files and unsubscribing.
depotcache and appcache
Two Steam cache folders that are safe to empty, what each one holds, and why you should not go any further into the Steam directory than these.
steamapps/downloading
Where Steam stages an update before it lands, why a folder there can outlive the download, and the one case where clearing it costs you real progress.