This list does not mention the most surprising (and a bit disturbing) bash feature for me: it can open tcp/udp sockets on its own  through a feature called “special redirects” (there’s no actual filesystem involved here):
$ (exec 3<>/dev/tcp/www.google.com/80 ;
echo "GET /search?q=bash+madness HTTP/1.0" >&3;
cat <&3 ) | less
I scanned over this, and OSH  should support basically everything here. If it doesn’t, feel free to file a bug!
Hm now that I look more at the repo, it looks like this would be a good test: https://github.com/dylanaraps/pure-bash-bible/blob/master/test.sh
Lots of things can already be done with portable sh code. typeset for example is quite powerful but mentioned nowhere in the repo:
$ var=VaL; typeset -l lvar=$var; typeset -u uvar=$var
$ echo $lvar $uvar
In fact, this builtin is often wrapped by the shell itself, at least in OpenBSD’s ksh:
$ alias | fgrep typeset
typeset is not posix
“portable sh code” != “it works on my openbsd”
Note how I did not speak of POSIX; you mistake the ksh example as general assumption.
My reply to pl’s comment tells you what I meant with “portable” (admittedly, wording was a bit misleading).
what’s your definition of portable?
What do you mean with portable sh code? I just tried it with dash and it has no typeset built in.
At least available in Bourne and Korn shell derivatives; (Debian’s) Almquist Shell does not implement that particular builtin.