How can I reserve an specified amount of space for root on macOS?
I want to stop user applications and myself from unknowingly making the disk full, and always have some free space available.
The easiest way I can think of would be to actually partition your hard drive (https://support.apple.com/kb/ph5845?locale=hu_HU). Put room for OS X and however much space you always want spare on one partition, then use the other for everything else.
This will require a full reinstall and everything will be erased, so you'd need to ensure everything was backed up. Don't partition your disk unless you know what you're doing.