Also of interest would be whether any popular x86 libc's actually leverage this already.

I did a quick skim (disclaimer it's sometimes hard to rule out that an SSE optimized version is in there somewhere).

* musl 1.1.16: no

* glibc 2.26: yes (also an avx2 optimized one)

* Apple Libc 1158.50.2: optimized (non-SSE2) versions of several str* funcs exist but strcpy() has an >=SSE impl