> However at the same time, not even offering a fallback path in non-assembly?

There's probably not any hand-written assembly at issue here, just a compiler told to target x86_64-v2. Among others, RHEL 9 and derivatives were built with such options. (RHEL 10 bumped up the minimum spec again to x86_64-v3, allowing use of AVX.)

Or even, a compiler told to target nothing in particular, and a default finally toggled over from "Oh, we're 'targeting x86'? So CPUs from the early 2000s then" to "Oh, we're 'targeting x86'? So CPUs from the mid-2010s then."