NetBSD can run on most Amiga's with a MMU and 24MB or better [1]. That's either an A3000 (1990) with memory expansion or pretty much any model with a 68020 + MMU + RAM accelerator board. I don't know if you could get it to run on an Amiga 1000, but certainly on a 500 or 2000. So a 1987 computer... Though most of the accelerator boards that are easy to obtain today are newly manufactured FPGA based ones...

(NetBSD will probably run on older hardware than that too - it's just I remember people running it on Amiga's, so it's the one I'm aware of)

[1] https://wiki.netbsd.org/ports/amiga/

I've always wanted to try NetBSD on Amiga but I sold my MMU-equipped accelerator long ago. It'd be symbolically nice: a Unix clone running on the Amiga and Workbench clone running on the PPC Mac...

There are completely new accelerator boards being brought out for Amiga regularly, so you still can...

Of course at this point most of them replace more and more of the original hardware, so there's not much left of the original that'll still be in use.