when IWD daemon is used, NM is able to operate fully without wpa_supplicant presence. So, only when iwd USE is not selected, wpa_supplicant should be required. Tested on ~amd64.
That's what the current NM ebuilds enforce wifi? ( !iwd? ( >=net-wireless/wpa_supplicant-0.7.3-r3[dbus] ) iwd? ( net-wireless/iwd ) )