Are plastic cups allowed in the cabin for takeoff or landing when the airline provides the cups?
The answer is found in the regulation and clarified in guidance. As you’ll read, the regulation and guidance require items provided by the certificate holder (airline) to be collected prior to taxi, takeoff, and landing.
(a) No certificate holder may move an airplane on the surface, take off, or land when any food, beverage, or tableware furnished by the certificate holder is located at any passenger seat.
Additional information about the stowage of service items prior to takeoff is found in this section: 3‐3549 Stowage of Galley Service Items.
There is a modest amount of content within the guidance to read, so it’s best to provide the link and not extract it here. This way you can read all of it and ensure you’re reading the most current revision of Order 8900.1.