I booted Debian Linux on a 4-bit intel microprocessor from 1971 - the first microprocessor in the world - the 4004. It is not fast, but it is a real Linux kernel with a Debian rootfs on a real board whose only CPU is a real intel 4004 from the 1970s. The video is sped up at variable rates to demonstrate this without boring you. The clock and calendar in the video are accurate. A constant-rate video is linked below.
This made me look up if you could somehow run it on vacuum tubes, sadly not
But https://www.linux-magazine.com/Online/News/Hacker-Successfully-Runs-Linux-on-a-CPU-from-the-Early-70s
9 day boot time on a 4004!
OMG!
https://dmitry.gr/?r=05.Projects&proj=35.+Linux4004
He eventually got it down to under 5 days!