If you mean a standard, it will be too abstract to be considered as something that can run anything. Sometimes it has to be too vague in order to not make existing Bourne-style shells non-compliant. This isn't, for example, a standard for the C programming language where you can throw some things into the UB bag or the ID bag. POSIX sh is more what you'd call "guidelines" than actual standard.

Even if it were an implementation, how would you write a "POSIX sh" equivalent of these lines taken from a bash script? (My point isn't that it's impossible.)

mkdir -p /tmp/nodes/{a,b,c,d,e} ; NODES=( /tmp/nodes/{a,b,c,d,e} ) ; echo ${NODES[$((`date +%-j` % ${#NODES[@]}))]}