Mac storage basics · Purgeable space and local snapshots
Will my Mac delete my files automatically when storage runs low?
Short answer: Not your documents. It frees purgeable items — snapshots, caches and local copies of iCloud files that stay in the cloud — which then download again when opened.
It won't delete your own documents. When space runs low, macOS removes things it has marked as purgeable: local snapshots, caches, and downloaded copies of iCloud files that remain in the cloud (if you use Optimise Mac Storage). Those cloud files then show as online-only and download again when opened.
If files seem to have vanished, check whether they're in iCloud Drive (online-only) or Photos with optimised storage. Anything in the Trash is only removed automatically if you turned that setting on.