i was testing low disk space like situation on a Mac Mini used as small home network storage/server, maybe ~ 1o years ago (running maybe Yosemite at the time or something similar) out of curiosity - so Intel mac and pre-APFS system

not sure it this was OS or filesystem feature, but it refused to allocate literally anything if free space reached ~ 100-500MB on system partition so it was being kept _always_ usable, even logging was denied IIRC