It is not an entirely unreasonable idea. If a system runs out of disk space, an unexpectedly large number of operations will fail. Which can make recovery more problematic than you would assume. If you can immediately recover some disk space and have breathing room, it could make the difference in restoring service.

Keeping a bit of disk reserved for recovery is extremely common with copy-on-write filesystems like ZFS & BTRFS. Even deletion takes some extra space, so without a reservation it's effectively impossible to delete any files from a full disk.

Or, you know, have alert on disk space like adults.

This is an alert that must be fixed immediately, with an escape hatch to fix it quickly in case you truly don’t have time to manage your actual files.

It is also trivial to set up, and does not require me to figure out how to set up an OS alert, or trust that whatever alert process is running. So it is an essentially fail proof alert that works the same on any OS.

What’s a good argument against it?

In my experience this makes the problem worse. People either compensate for it, or stop trusting clocks at all. Usually a mix of both of those resulting in even less punctuality.