@lou It's detailed in the 1st post where the Stretch image is posted:

WiiFi is disabled until wireless regulatory domain is set (Pi 3 B+ only)
The domain can be done through 'Raspberry Pi Configuration' (rc_gui),
raspi-config or by setting 'country=' to an appropriate ISO 3166
alpha2 country code in /etc/wpa_supplicant/wpa_supplicant.conf.