The moment you start talking about C compilers you've stopped talking about most software running today on Linux systems. The vast majority of packages are delivered as apt/rpm/apk etc installers, which are overwhelmingly binary or interpreted code. If you're delivering an apt package, you will want to assume (or ensure through declared dependencies) some things about the target system, such as bash or Python.

And you'll find numerous bash-only scripts online, either as examples, helpers, stack overflow answers, and many others. Comparatively few bother to support POSIX sh - though that trend is somewhat reversing with the rise of containers and Alpine Linux and BusyBox with them, plus Apple's moves against bash.

Just like you will find no tutorials telling you to edit a file with ed, even though that is the standard text editor.

Finally, if you think it's in any way common to write software to support any POSIX-compatible system, or even any Linux system, you live in a much nicer corner of the world than I do.