Currently   os::Bsd::available_memory()   returns on macOS 
vmstat.free_count * os::vm_page_size();

But just using free_count is rather conservative because there are other memory categories on macOS that can be made available too  :
https://github.com/openjdk/jdk/pull/25384/files
// Available here means free. Note that this number is of no much use. As an estimate
// for future memory pressure it is far too conservative, since MacOS will use a lot of unused memory for caches, and return it willingly in case of needs.

See also the discussion here : https://github.com/giampaolo/psutil/issues/1277
At least (parts or all of) the other memory categories  inactive_count and maybe speculative_count  .

Looking at the HS API description 
https://github.com/openjdk/jdk/blob/428d33ef3ca0af34d8f164fe9d9b722e81e866a7/src/hotspot/share/runtime/os.hpp#L336
we want to have a  distinction between "available" memory and "free memory" so adding freeable categories might make sense .


There is a bit of additional information here https://developer.apple.com/forums/thread/118867
that confirms that free memory / free_count is only a fraction of what is available :  "Specifically, any report on the amount of free memory is unlikely to be useful."  .