Must be pretty recent. However, almost anyone who regularly went understood that there was a uniformly applied unofficial policy. I'd brought in stuff from McDonald's. I'd spilled my own can of Coke and had dropped some snacks on a ride. A CM came right to me and said she'd have it cleaned up but said nothing about the beverage. When I dropped the snacks a CM tossed them to me while on the Haunted Mansion vehicle.
And I remember there was an official policy on coolers brought into the parks being no bigger than necessary to hold a six-pack. I actually have this tiny cooler that I brought with me and had no issues.
The one uniformly applied policy was that if you looked like you brought a picnic in they'll tell a guest to stop. And separate sandwich components weren't OK, but already prepared sandwiches were. I wouldn't be surprised if that would still be enforced, with or without any official policy on it.