@flameeyes it doesn't look like anyone ever suggested the Austin Group add strcasestr to the standard, so I went ahead and did that now:
https://austingroupbugs.net/view.php?id=1818
Of course, they're busy finalizing the POSIX.1-202x / UnixV8 / XPG8 revision right now, so it would be years before this could make the version after that.