For essentially any other language this would be an absurd thing to say. For those languages you wouldn't consider a feature added until it was supported by the reference compiler
If you are using GCC, Clang, or MSVC then the standards support is excellent[1].
But putting that aside
> For those languages you wouldn't consider a feature added until it was supported by the reference compiler
Well, there is no reference compiler for C++. I don't think any language with either an ISO or ECMA standard has a reference compiler or interpreter.
There are gaps in support here in and there. But I would still say those mainstream compilers "support" C++17 for the same reason I would say that Chrome "supports" CSS3, despite it not implementing the full specification.
For essentially any other language this would be an absurd thing to say. For those languages you wouldn't consider a feature added until it was supported by the reference compiler