That's cool! Is that a GNU tar only thing? Based on it being a longopt, I'm guessing a GNU tar only thing. That's the problem with these things, it takes a while to get pushed to all the installed copies of tar running around. Perhaps it's time to check:
* MacOS Sonoma(14.6) has tar --auto-compress and --zstd
* OpenBSD tar does not appear to have it: https://man.openbsd.org/tar
* FreeBSD does: https://man.freebsd.org/cgi/man.cgi?query=tar
Not quite fully baked yet.