There are still Linux distros that will work on it. Adelie should. I think Arch32 as well. And Tinycore.
I am not as sure about Slackware but I believe the single-core (non-SMP) kernel runs on Pentium.
PS: Thanks to Peter Tribble for providing this system.
Edit: I've just downloaded the basic (Tribblix 0m40) iso, dd'ed [see below] it to a smallish USB stick and booted an old Thinkpad. Boot succeeded and I was able to log in to the minimal live session. Haven't done more than that yet.
# dd if=tribblix-0m40.iso of=/dev/sdb bs=1MI would have expected OpenLook. Xfce is ugly.
XFCE is much more functional than CDE.
On a personal level I'm impressed and fascinated by the fact that apparently one man created and has maintained an illumos distro for many years;
* making an OS distro at all is hard
* making an illumos distro is harder (less precedent to work from, and IMHO Sun didn't do a great job documenting things if you weren't inside Sun)
* making a different distro is harder; this isn't an OpenIndiana rehash, AFAIK it's mostly novel
* and of course maintaining it for so long is a huge undertaking
Retro will never die.
I tried booting various Illumos distros through USB sticks on two different AM5 computers, and it got stuck very early on. I assume due to some incompatibility with USB 3.0. Meanwhile, a friend of mine booted on a Thinkpad just fine from a DVD.