Large projects like the Linux kernel, which are mostly C, use the preprocessor in .S files quite heavily.

Linux has hardly any .s files.

  ~/linux$ find . -name '*.s'
  ./arch/x86/kernel/asm-offsets.s
  ./kernel/bounds.s
  ./scripts/mod/devicetable-offsets.s

But there are plenty of .S files with #includes

  ~/linux$ $ find . -name '*.S' | xargs grep '#include' | sed 's,:.*,,' | sort -u | wc -l
  1217

FYI, `grep -l` will print just the matching filenames, no need for the sed/sort -u. `git grep` and `rg` also support this option. (I say this because I also used to overcomplicate this in my typical usage.)

Many such cases.