Homebrew 30d says 27% Intel
My gut says 20-25% is a reasonable ballpark.
Apple generally provides 6-8 years of OS support for hardware. My 6.5 year old 2019 Macbook Pro works fine with macOS 26; macOS 27 will drop support in September 2026; macOS 28 will drop Rosetta Intel support. macOS 26 get updates thru September 2028 at which point all Intel mac hardware will be >8 years old.
[1]: https://store.steampowered.com/hwsurvey/processormfg/
[2]: https://formulae.brew.sh/analytics/homebrew-os-arch-ci/30d/
Neither statistic comes from a sample group that's more representative of the average user, who is mostly browsing the web and using a handful of undemanding applications, and is therefore less motivated to upgrade for performance reasons.
But the MacOS support at this point is pretty atrocious. I mean, it technically runs, but the OS is clearly designed in a way that ruins performance on this hardware. It’ll run Vim, sure, for 2 hours on battery, hot as hell. At this point, Intel Macs are only fine for running Linux.
PS: and on my Win11 machine Firefox says that I'm on 'Windows NT 10.0', which AFAIK is the same NT version that was also reported on Win10.