OpenBSD and the rest have a limits file where you can set RAM limits per user and sometimes per process, so is not a big issue.

On GNU/Linux and the rest not supporting dynamic swap files, you can swap into anything ensembling a file, even into virtual disk images.

Also set up ZRAM as soon as possible. 1/3 of the physical RAM for ZRAM it's perfect, it will almost double your effective RAM size with ease.