Lots of things in C/C++ are nonstandard or optional, like the existence of optimization flags. Nevertheless, it's supported by literally every compiler I can think of for at least the last decade. I had to get into weird interpreters written in Python for university projects before I found anything that didn't support it.

Plus, it's nicer to read than #ifndef FOO_BAR_BAZ_PROJ_DIR1_DIR2_DIR3_FILE_H

#endif /* FOO_BAR_BAZ_PROJ_DIR1_DIR2_DIR3_FILE_H */

On every file.