I’ve been using a lightning adapter (to 30-pin) with a Wahoo key essentially since the iOS TrainerRoad app was first in beta. I can’t recall all of the iOS versions I used it on and it has generally worked well without issues. Occasionally I saw the “ANT+” indicator in the app not show as active, but messing with plugging it in and out or killing and launching the app again seemed to fix it.
I was on an iPhone for about 3 years until switching to an Xr in the last few weeks. The key worked on the 6s and now on the Xr as expected. My wife also picked up TR again in the last few weeks and uses the same Key when she rides with her Xr. We’re both on iOS 12.1 right now.