Case in point: strncasecmp is supported everywhere now (I can't confirm AIX or HP-UX, though). Times have changed. Conformance is heads-and-shoulders better than it was 10 years ago.
systemd/Wayland is at the opposite extreme. They refuse portability not because of trivial issues, but because their favorite kernel features don't exist everywhere else. They've chosen a no-compromise approach.
What's often left out their diatribes about the BSDs is that they often have features which Linux lacks, or implement some features in superior ways. Yet they're fine settling with the Linux deficiencies.
At the end of the day systemd is Linux-only because that's a simple decision they made. It was their choice. But to compare their gripes with the nightmare that was Unix compatibility 10 or 20 years ago is just silly.