There really isn't a POSIX sh manual page; and you are conflating Bourne with Bourne Again on RedHat.

The closest that anyone comes is, I believe, OpenBSD. The ksh(1) manual page is the PD Korn shell manual. And the separate sh(1) manual page "describes only the features [of ksh] relevant to a POSIX shell".

On others, sh(1) is usually the manual page for one of the named shells.

Debian and Ubuntu's sh(1), for example, is the dash(1) for the Debian Almquist shell, which is famously not a POSIX sh, because it explicitly includes 3 extensions that Debian people could not bear to part with in their big project of over a decade ago to remove bashisms from package maintainer scripts and suchlike.

If the default sh is set to bash (yuk!) you will need to use ash or dash to get the basic POSIX shell and man page. On my Xubuntu 18.04 system, "man sh" brings up the dash man page.

Thanks for nothing RedHat & Apple!