There’s a bit of history here - the splitting of CSRs was initially messy - sometimes there wasn’t an extension name to describe the CSR, or there were later changes that would be backwards incompatible if we were gating CSRs on a particular extensions (e.g. zicntr, zihpm). Our current behaviour is documented here User Guide for RISC-V Target — LLVM 17.0.0git documentation
I think gating CSR names for more recent extensions would likely be fine (and perhaps a minor improvement), but it hasn’t been a priority for anyone.