Description:

iwd can work as alternative to wpa to some extend with comprehensive network managers like connman or networkmanager (by iwd's arch wiki page)

But users who installed those managers by the name would pull in wpa as dependency by default.
And users can only get rid of that by -Rdd wpa_supplicant afterwards.

IMO iwd and wpa_supplicant should provides the same virtual package for user to choose from, like the case for libgl and some other multiple backend solution.