Sounds to me as a Linux specific, special purpose call, though I don't really understand why they did this. Specially the stx_ prefix of fields seems to indicate that this really isn't meant as a compatible stat.
You can call syscalls over the syscall unit, but what do you really need that compatible cross-unix stat/baseunix.fpstat doesn't give?
To me this reeks of the horrors of stat32/stat64 that needed linux specific ifdefs for a decade.
The only thing one commonly would need is higher resolution timestamps, but applying this to the compatible (fp)(f)stat has been delayed, see point 8 here:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a528d35e8bfcc521d7cb70aaf03e1bd296c8493fAll the other features seem to be for specific cases like network daemons (nfsd,smbd), and management tools of newer FS.
But maybe it is logical to first test the waters with an extra call, and only later go through the massive undertaking of updating the portable call.