https://pubs.opengroup.org/onlinepubs/9699919799/utilities/cat.html#tag_20_13
POSIX compliant cat only supports the "u" option.

u = "Write bytes from the input file to the standard output without delay as each is read."

On GNU/Linux, the options are AbeEnstTv, with e being a shorthand for vE, and u is being ignored, making GNU/Linux already POSIX non-compliant as it does absolutely nothing.
Among other options are the usual "help" and "version, which on both FreeBSD and OpenBSD are absent, and thus you can only see from the man pages.

On FreeBSD, the options are belnstuv, u being "Disable output buffering.", which is again POSIX non-compliant as it does something else than what is should do.

On OpenBSD, the options are benstuv, u being "The output is guaranteed to be unbuffered (see setvbuf(3)).", it's POSIX compliant!

On Plan9, the options are mn, no mention of u at all, so POSIX non-compliant.

And finally on Busybox (think Alpine, postmarketOS, and Android), the option is only u.......except it's ignored, so again POSIX non-compliant...

So cat from 5 major POSIX coreutils have been compared, only 1 of which is actually compliant.
Sad...