The builtin translates to dispatch_threads_per_threadgroup, which is available on "All OS: Since Metal 1.0" according to the Metal language spec. However, actually using it gives me the following error:

:12:19: error: 'dispatch_threads_per_threadgroup' attribute requires Metal language standard macos-metal2.0 or higher
, type2 wg_size [[dispatch_threads_per_threadgroup]]