`CXX_EXTENSIONS OFF` does not turn off extensions in GCC 11

Using CMake 3.20.2; this has been described here: https://stackoverflow.com/questions/67641124/gcc-11-how-to-tell-cmake-i-dont-want-the-default-c-gnu-extensions

GCC 11's default ISO C++ standard is C++17, so that when I declare target_compile_features(mytarget cxx_std_17) CMake (correctly) does not generate any -std flag. However, GCC's default is actually -std=gnu++17, and if I want -std=c++17 I should be able to declare set_target_properties(mytarget PROPERTIES CXX_EXTENSIONS OFF). Unfortunately, this has no effect, and CMake still omits the -std flag.