The Intel 80486DX came out in 1989, and that can run Gentoo Linux. This is really nice because most Gentoo portage packages are very current. You can build a completely current Linux kernel for it too.
What makes this possible on Gentoo is that the installer software is compiled for the old 80486DX, and then during install you can recompile the portage software packages for your target architecture (i486, i586, and i686 for 32 bit machines). You do have to edit some of the config files, as they are set for i686 by default.
The reasonable way to do this is to install Gentoo using a fast machine onto an older hard drive, and then move the hard drive onto the old machine once done. There are some cheap USB-to-IDE adapters you can get for around $20 to do this. Recompiling on an old computer is painfully slow, even with large amounts of memory.
The software patches needed at present are listed in this posting:
Another subtle problem is that the BIOS on such old computers often have bugs, so using the GRUB or Legacy GRUB bootloaders don't work. I have the most success installing the ancient LILO bootloader. It isn't pretty, but it works best on such old hardware.
People just marvel at me at work with my Compaq LTE Elite 4/75CX running the latest Linux kernel and current Gentoo userland.
Love that built-in mouse, eh? Well, it was cutting edge for back then.
I previously used the old Debian 'Squeeze' distribution (2011) because it supported 80486, but it has become too dated. That was the last Debian version to support 80486. Understand, the Linux kernel still supports 80486, but the distributions and their userland utilities have been dropping support for i486 and often i586.