I updated to the latest Spotify iOS app which claimed to fix this issue: https://community.spotify.com/t5/Ongoing-Issues/Airport-Express-Not-In-Spotify-Devices-Menu/idi-p/15...
Several weeks ago it was working fine. My Airport Express no longer shows up in Available Devices on my iPhone 6 Spotify app. I have deleted and re-installed the Spotify iPhone app and restarted the iPhone6. My iPhone 6 is not in Low Power Mode. When I goto the iOS Spotify, Settings, Devices Menu, all it shows is: LISTENING ON This iPhone. If I turn on my Marantz receiver, then it (Marantz SR6009) shows up as a Spotify Connect device.
iPhone6: iOS 10.2.1
iOS Spotify App: 188.8.131.523
Airport Express version: 7.6.8