[dfawcus]

yet if one does that, and has suitable warnings turned on, GCC will complain:

    array2.c: In function ‘main’:
    array2.c:25:17: warning: passing argument 1 of ‘arr_fn2’ from incompatible pointer type [-Wincompatible-pointer-types]
       25 |         arr_fn2(array);
          |                 ^~~~~
          |                 |
          |                 char *
    array2.c:13:22: note: expected ‘char (*)[15]’ but argument is of type ‘char *’
       13 | void arr_fn2(Arr_15 *arr) {
          |              ~~~~~~~~^~~

The above was -Wall -Wextra

Or are you just referring to the function where one defines it as apparently 'pass by value'?