It is not retro-computing. New 32-bit and x86 CPUs are produced, sold, and used today.
See (relatively recent) list of manfuacturers here:
https://en.wikipedia.org/wiki/List_of_x86_manufacturers
and scroll down for other categories of x86 chip manufacturers. These have plenty of uses. Maybe in another 30 years' time they will mostly be a hobby, but we are very far from that time.
Which ones of those don’t have MMX / SSE? Debian is not enabling any x86 instruction extensions that didn’t ship on the Pentium Pro
This is not accurate, the Trixie 32-bit release requires a Pentium 4.