> suppose the system firmware itself just doesn't know what to do with them.

Anything since socket 1155 can work and even Westemere/Nehalem should work too, except you don't really want a system that old.

I have a 4Tb M.2 drive in x1 slot in my GA-Z77X, works fine.

Not a boot drive and I didn't bother to look if the BIOS supports booting from NVMe, though a quick search says the support for that was started with Z97 chipset/socket 1150.

> use a little sata SSD as /boot

For Linux you can even use some USB thumbdrive, especially a small profile like Kingston DataTraveler Micro G2.

https://www.aorus.com/motherboards/ga-z77x-ud3h-rev-10/Key-F...