We also had (well, have) a Unix-like extension for the shipped TOS/GEMDOS/GEM OS in the form of MiNT/FreeMiNT, which went on to form the foundation of the official MultiTOS which unfortunately died along with the ST when Atari Corp died.

It had/has a POSIX(ish) API, device files, mountable filesystems, pre-emptive multitasking, TCP/IP, etc. while still being able to run classic TOS binaries.

You can run this in an emulator or on hardware still today and it still gets active development, under the GPL.

e.g. https://www.youtube.com/watch?v=GOkDuLmgWFo

That looks like it would be a ton of fun to try. I wonder how that would run on my MiST!

quite well. Though it benefits from a faster machine with an 020 or 030 and more memory.