A lot of hosting companies run KVM/qemu/Xen or even UML with 32-bit guests by default. But yeah in terms of physical computers 64-bit has long been the default.

I remember upgrading from 32-bit to 64-bit in-place a few times, which is a bit fiddly but not impossible with Debian. I wrote up a couple of guides once upon a time, but right now I can just find this old wiki-page discussing the process:

https://wiki.debian.org/Migrate32To64Bit

Sure but cheap Intel Atom N270 netbooks are probably still being used in poor countries even though they were released around 2008/2009.