Marginally more researched speculation: if someone builds wheels with their distro flags incorporated they’ll start building v3 wheels once their distro drops support for v2 hardware (I haven’t checked, but it wouldn’t surprise me if Arch or another leading edge distro was already assuming v3)
We also can’t rule out gcc distributors bumping their default x86-64 arch up to v2 (or even v3) instead of keeping their own default build flags elsewhere:
I don’t know if there is an easy way to get this kind of target info out of ELF binaries (short of scanning every opcode looking for the newer ones)