Real question though: who's gonna run a CI farm of old hardware? That sounds not-cheap and commercially untenable.

I imagine you don't need to; you can emulate i586 on x86_64, and it would probably be performant enough.

But I suspect that's not really the hurdle: none of the existing Debian developers care enough about it to maintain it, and no one who cares about it enough about it is willing to maintain it.