POSIX supports the export foo=bar syntax:
Allowed by POSIX, but not some Unix shells (Test with osh, ash386). Warn user of this construct.
the point of this script is to check for bashisms which is typically defined as "not in POSIX". what you describe is "portable shell" which is an ugly/undefined subset of POSIX that no one really knows and is often times different for every person. how far back exactly do you want to run your compliance ?
imo, it has no business being in checkbashisms as it should only be testing for POSIX compliance. if a shell isn't POSIX compliant, it's garbage and should not be used.