Is there a publicly available testsuite for compliance with recent versions of POSIX? That seems like it’d be the most systematic route to improving compatibility, but I can’t find one.
I think that’s a sad case of no one really caring. At least not enough to actually do it. Though to be fair it’s a massive undertaking, such a test suite would likely be incomplete and rife with bugs.
Digging around a bit, there used to be something called the Open POSIX Test Suite, but it has no updates since 2005, so wouldn’t cover the POSIX.1-2008 features even if it had been complete at the time (which it wasn’t).