Cannot get pins on esp 8266 adafruit wifi module to read HIGH on pin 15
Pin 15 is one of the boot configuration pins (0, 2, 15) and must be LOW at boot. To ensure it, dev boards with esp8266 have external pull-down on pin 15 (and pull-up on 0 and 2). To read the pin HIGH you must overcome the pull-down.

Adafruit Feather HUZZAH has 4.7kOhm resistor for the pull-down, so it is a 'strong' pull down. NodeMcu has 12kOhm resistor on io 15.

From schematics of Adafruit Feather HUZZAH



Note: setting pin as OUTPUT and HIGH doesn't make it read HIGH in code. Only INPUT pin state can be read with digitalRead (only on some MCUs digitalRead reads the same register set by digitalWrite).