Well x86 at one point, arm both the 32 and 64 bit versions. I think they had RISCV support in their source tree at one point but not really at a commercial level. It does cover a lot different levels of hardware though
PPC32/64 of course, and for a long time Darwin still contained remnants of its predecessor's support for SPARC, PA-RISC, and m68k.
Which Apple products run arm32 XNU? Their first Apple Silicon CPUs were already arm64.
Does Apple use macOS in servers in its datacentres? Or are they all Linux?
Surely at a minimum they need macOS for CI.
Apple does have one advantage here-they can legally grant themselves permission to run macOS internally on non-Apple hardware, and I don’t believe doing so legally obliges them to extend the same allowance to their customers.
But that might give them a reason to keep x86_64 alive for internal use, since that platform (still) gives you more options for server-class hardware than ARM does