> most portable os
Eh... I think the Linux kernel + your choice of libc/userland has it beat these days.
Modern Linux can't even scratch a 486 and some Motorola platforms. Or VAX. Heck, I run NetBSD 10.1 vanilla under simh 3.8 for 9front emulated on an amd64 laptop (old Celeron, 2GB). Slow, but enough to play Slashem.
On portability on compilers, plan9/9front it's unbeatable. Do you now Go compiling from any OS to any arch? The same here, but just for an OS obviously. Albeit I can still run Golang under i386, and tools like Rclone under 9front i386. That's really cool.
Modern Linux dropped support for a lot of old and niche CPUs.
The Linux kernel dropped 386 support fourteen years ago.
https://www.theregister.com/2012/12/12/linux_no_longer_runs_...