Собственно, сабж: https://www.phoronix.com/news/Intel-i486-Linux-Possible-Drop .
So I *really* don't think i486 class hardware is relevant any more. Yes, I'm sure it exists (Maciej being an example), but from a kernel development standpoint I don't think they are really relevant.
At some point, people have them as museum pieces. They might as well run museum kernels.
Moving up to requiring cmpxchg8b doesn't sound unreasonable to me.
