The degree of $'...' support also needs to be taken into consideration when porting. The POSIX Folks' proposal to put this in POSIX sh mentions one in particular:
stephane: ksh93 is the shell $'...' comes from (while
$'\uxxxx'[and$'\Uxxxxxxxx'] comes from zsh: http://www.zsh.org/mla/workers/2003/msg00223.html) [^]
From what I got here on my Debian bullseye, the ksh2020 understands $'\U1F600'. This is the only "official" Korn shell I can get on this new distro.
mksh parsed it but botched it entirely with a U+FFFE. Since it did not complain about a syntax error there's gotta be something wrong with it's understanding of Unicode. It handles $'\U01F60' just fine.