Most gas powered drones don’t actually use much fuel and thus their oxygen use is limited. At the extreme end a 20 pound drone might use the same oxygen per minute as ~5 people and you’re not going to stay in the same place for that long vs people going through rough terrain. If anything adding gas sensors would more than make up for such issues.

Personally I’d be more concerned with explosive gas mixtures, but that’s not an issue in most cave systems.