Pretty sure the policy is similar to what major music festivals have so they can keep people from bringing alcohol in.
However, in a tradition as old as time, the website has not been updated and says the following:
We offer a variety of fresh, delicious food for our guests to enjoy at restaurants and concession stands throughout our resort; however, we understand that guests may need to bring outside food and drink into our parks. Please note the following guidelines for bringing food and drink into our parks:
Acceptable items:
- Bottled water
- Small snacks that do not require heating
- Any food required for medical purposes and medically–indicated nutritional supplements
- Any food required for special dietary needs
- Baby food/baby formula
- Soft-sided insulated bags no larger than 8.5" wide x 6" high x 6" deep
Prohibited items:
- Picnic lunches
- Food that requires heating or refrigeration
- Alcohol and glass containers
- Hard-sided coolers
- Soft-sided coolers larger than 8.5" wide x 6" high x 6" deep
- Coolers, suitcases, and bags with wheels
- Folding chairs
- Clothing with offensive language or content
- Clothing that represents someone as emergency personnel
Please be advised that all bags and personal items are subject to inspection.
https://www.universalorlando.com/Resort-Information/What-to-Bring.aspx