Hola Julia

In this sentence:

"Las que comimos en ese restaurante, estaban deliciosas"

we are using estar because when we talk about "taste" in food (or drink) we use "estar". When you are in a restaurant and you want to tell the chef the food is delicious you can say "Está delicioso" or anything related to the taste of the food, also negative: "está salado" (it is salty), "¡está asqueroso!" (it tastes horrible!)

If you used ser, instead of estar to talk about food, you will be referring to a permanent quality of that food, for example: "Las naranjas de Sevilla para la mermelada son muy amargas" (Oranges from Seville [used] for marmalade are very bitter) or "Los plátanos son dulces" (Bananas are sweet)

In the first examples with estar, you are saying what taste the food has at the moment of tasting it, not as a permanent quality.

Hope this helps,

Saludos

Inma