illumos gate

system(3C) and possibly similar code contains conditions such that in a posix environment it runs the POSIX shell (/usr/xpg4/bin/sh) and in others, the regular shell /bin/sh (formally the bourne shell).

On Illumos systems, these are now both ksh93, it is very likely that the conditional logic can be removed, avoiding, for eg, #845.

Someone would need to verify that ksh93 is not doing anything fancy in a way that alters its behaviour when invoked as /usr/xpg4/bin/sh.