Option to disable "Are you still watching?"

Are you referring to Netflix? It specifically asks “Are you still watching?”

If you are using Roku devices, they have a similar feature they call “Bandwidth Saver”, but it uses different language. (And can be disabled.)

I’m not aware of a Plex message with that wording. Can you share the device being used, and perhaps a screenshot or picture when it occurs?

A similar feature in Plex is called “Post Play Auto Play”, controlling automatic start of a new episode when one finishes.

Play Queue Post-Play Screen | Plex Support

That feature has episode/duration/time limits. If that’s what you’re referring to, there’s a popular feature suggestion to remove these limits: Continuous Autoplay