What conditional compilation is needed around C++ attributes?

I'm starting to add [[nodiscard]], [[noreturn]] and [[fallthrough]] to my code base, but I still worry the code might need to be used on an older C++ version.

I'm only worried about g++, clang, and Visual Studio.

To be safe should I make every single usage depend on a version check for the exact version it appeared? Or, do these compilers accept (or does the spec even mandate they accept) any [[attribute]] even if unknown, starting with C++11?

Were these allowed and ignored (or supported) even before C++11? (I ask not what the spec says but what the software actually did.)

#if __cplusplus >= 201703L
#define MyNamespace_NoReturnCPP17 [[noreturn]]
#else
#define MyNamespace_NoReturnCPP17
#endif

(Off subject but if I were on the C++ committee, I'd re-use the case keyword to do the same thing as switch, when followed by (, except it would in effect have an automatic break before each case label. And within the case's block (and nowhere else), fallthrough would be accepted as a keyword, that suppresses that automatic break.)