Please unset this function after use, since it's not useful for the user's environment.

Alternatively, just write the code in line and unset dir and file afterwards.

The only purpose I can see for the :- assignment is if the variable went into the environment (preventing indefinite recursion if /etc/profile is copied to /usr/local/etc/profile), but that is not how a variable assignment before a function call works in our sh. (It does work that way in bash and mksh.)

In general, the exact effect of variable assignment before a function call is inconsistent across shells and it would be good to avoid breaking users of strange shells by putting unusual code in /etc/profile.

So if you want to guard against indefinite recursion, the code could be

_loaded=${_loaded:-/etc/profile}
export _loaded # also avoid export name=value syntax
_load_profile /etc /usr/local/etc # or inlined code
unset _loaded
unset -f _load_profile