Have you ever tried to make a portable C application using multiple C compilers (not gcc or clang) across multiple OSes (not only POSIX)?

It is lots of fun with extensions not covered by ANSI C and compiler specific behaviors.

I have - although POSIX covered a fair number of them and POSIX-ish or pretend-to-be-POSIX made up some more. That could all usually be covered with a pretty thin utility layer, and the compiler-of-the-day for whatever the platform was. (and, sometimes, a whole lotta #IFDEFs!)

You must be talking about something more extreme, maybe in embedded land? Do you happen to have a small number of f'rinstances?

Experience in the late 90's, early 2000's, porting C code between multiple systems.

Yes. It's actually easier for me than C++ because the language stopped changing. You can get by with C89 for 99.9999% of C code bases out there.

C'89 is an awesome language (and language standard). The document is readable and one can reasonably expect to understand the whole language, dark corners and all.

(Features in a language that one doesn't understand are dangerous. Nobody understands all of C++17. I'll let you reach the conclusion.)