> whether that's a thing we care about today in 2025

Good question actually! ARM64 has a proper CAS it seems, but I think smaller ARMs still have only LL/SC and that's still relevant for embedded. I can't find a definitive answer for POWER, it is possible that is till only has LL/SC (I leave it to you whether POWER is still relevant, but certainly IBM cares that the standard support it, and it was still relevant during C++11 standardization). Can't find a definite answer for RISC-V: it seems that originally it only had LL/SC, but there are AMO extensions that add RISC-V.

I think most GPUs have native CAS instructions.

There are probably other embedded processors that are still relevant for C++, who knows what they support.