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Debian Trixie raises x86 minimum requirements to i686.
(www.debian.org)
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Lots of expensive industrial equipment runs these kinds of processors still. You can still buy motherboards with 8 bit ISA slots even, although you'll pay quite a premium.
But all of that kind of gear typically runs its own distro with an in-house build system. For example, my work uses a flavour of Buildroot for their embedded Linux systems and you can just set whatever processor type you like all the way back to plain old i386 when you build it.