My Pluto app on my apple TV device has lost the on demand icon.

As mentioned above,

• If the issue still persists, then contact the app developer for further assistance.

Apple TV 4K, tvOS 13

If that is accurate, then update your Apple TV system software. The current version is tvOS 14.7, as of July 19. Update your Apple TV - Apple Support

Also verify that automatic app updates is activated, as that may be needed to keep things running as intended. Set up automatic updates or installation - Purchase and download apps on Apple TV - Apple TV user guide - Apple Support

Try this for any misbehaving app:

• Force-quit the app, then relaunch and try again.
• If the issue persists, force-quit the app, restart the device, then relaunch and try again.
• If the issue persists, delete the app, re-download and try again.
• If the issue still persists, then contact the app developer for further assistance.