How many years ago was that? POSIX compliance has come a long way and most of the proprietary vendors (the ones with all the corner cases) are gone. These days not only do platforms like AIX and Solaris have much fewer corner cases, they're even adopting Linux and GNU extensions wholesale. Anyhow, most people can ignore these altogether. Portability between Linux and the BSDs is much easier. macOS is the biggest outlier in terms of corner cases yet in many ways the best supported thanks to the popularity of Homebrew.

C++ is a different matter, but C++ portability is a headache even if you stay on Linux. Likewise, trying to maintain OS-level portability of monolithic codebases between Windows and Unix is a fools errand, which is why Windows Subsystem for Linux (WSL) is likely to only get better.

How long ago? 15 years - and it was at least a decade-old code base. But I had to take some new code that was something like Windows-and-Sun-only, and port it to run on all of the other architectures.

Yes, cross-platform development is more work, but that example you gave is the wrong way to do it. It's better to abstract the differences using typedefs, functions, macros, etc., and keep the platform switches in a few isolated places in the code.

Single platform cross-development (e.g. iOS) is a lot easier.