There is definitely no AVX-512 support on my CPU. Which is also true for most of my users. I don't bother with AVX-512 for that reason.
Another substantial population of my users are on aarch64, which memchr has optimizations for. I don't think StringZilla does.
This is exactly my issue with targeting AVX-512. It isn't just absent on "older AVX2-only CPUs." It's also absent on many "newer AVX2-only CPUs." For example, the i9-14900K. I don't think any of the other newer Intel CPUs have AVX-512 either. And historically, whether an x86-64 CPU supported AVX-512 at all was hit or miss.
AVX-512 has been around for a very long time now, and it has just never been consistently available.
But its fair to say that I’m mostly focusing on the datacenter/supercomputing hardware, both on the x86 and Arm side.