Why is Reddit not banned from Google search? Showing Google a different page to a human mobile visitor gets websites blacklisted from the Google search indexes. Are they too big to get banned?

This is a good point. That's pretty much what is happening. Any result you click prompts you to use the app after a few seconds with the only way to get around it being to "use desktop" which most people probably don't know.

Google allows you to show them your full content while only showing normal visitors less. It's when you show completely different content that you get banned. Paywalls are allowed.

I recommend using slide on android: https://github.com/Haptic-Apps/Slide

There are also other apps to choose from, just search “reddit” on F-Droid.