[libc++] Unsupported Tests

Is a test classified as "unsupported" always when the library is ahead of the compiler? For example: the nodiscard attribute can be present in the library and at same time may not be implemented in the compiler.

Yeah, that can be one reason why a test is marked unsupported. Although if we expect the compiler to catch up, those tests are often marked as XFAIL so they start “failing” when the compiler implements the feature.
The most common reason a test is unsupported is that it requires a certain dialect (Ex. C++17), so the test is marked // UNSUPPORTED: c++98, c++03, c++11, c++14