This metric isn't really useful. This disregards any value of say a compiler implementing 99% of all features ever announced, including draft C++26 features, because it's missing a single C++17 feature preventing it from saying it's fully compliant so we shouldn't bother reading about anything but old stuff.
The big 3 already support features from C++23 worth learning about https://en.cppreference.com/w/cpp/compiler_support Obviously you can't always use that, but most places with a C++ compiler today support at least some useful features past C++17.
Your comment seems to have split in two, so I'll respond to this one as it's the longer of the two.
I agree it is annoying when compilers don't support the same features but my point is the question isn't whether there is an unimplemented feature from that revision in many compilers it's whether the feature you want to use is commonly supported. As an example, if you want widely implemented features like <=> from C++20 then it doesn't really matter most compiler stdlibs don't support riemann_zeta from C++17. Waiting for them to do so only sets you behind years or decades because you're looking for arbitrary features you'll likely never use to be universally supported too.
The big 3 already support features from C++23 worth learning about https://en.cppreference.com/w/cpp/compiler_support Obviously you can't always use that, but most places with a C++ compiler today support at least some useful features past C++17.