Disney says
Impossible™ Patty, Dairy-free American Cheese, Caramelized Onions, Lettuce and Tomato on an Allergy-friendly Bun with Cuties® Mandarin Oranges or Steak Fries
Friendliest.app says
The Impossible burger can be served on an allergy-friendly bun that is free of gluten/wheat, dairy/milk, fish, shellfish, peanuts, tree nuts and soy.

Steak fries are cooked in a dedicated fryer to reduce cross-contamination.

This meal contains eggs (bun) and soy (steak fries). It can be made soy-friendly by selecting mandarin oranges instead of steak fries. 

If ordered with a sesame seed bun this meal is egg-friendly and plant-based, but contains other allergens.