Spotify works absolutely fine for me in Airplane mode. The reason I pay for Spotify is because it is the nest solution I have found for easily curating music for offline playback since I don't have unlimited data and I'm often outside of good coverage areas.
It certainly isn't perfect and there are aspects of the UI I find frustrating, but offline access and playback are the features that seem to work the best.
Edit: I also don't use Spotify for any podcasts and use other software since there are lots of great options for offline podcast listening that are way better than Spotify.
I don't doubt that there are phones out there for which airplane mode works with Spotify. It's often the case in software that the bug is just not encountered, for some.
But it was also just an example of the many ways the app is broken here and there. Freezes up, crashes, etc...
And this I've gotten across phones.
But that's just the bugs. As others have said the whole thing is not even close to designed for podcasts.
Podcasts are an entirely different use case, that needs to be RSS-shaped, not like music.
But you make it sound like it's a widespread problem. For me their app is just great and much faster than Apple Music that may randomly fail at playing some track. And it works on airplanes just fine.
It certainly isn't perfect and there are aspects of the UI I find frustrating, but offline access and playback are the features that seem to work the best.
Edit: I also don't use Spotify for any podcasts and use other software since there are lots of great options for offline podcast listening that are way better than Spotify.