Is it necessary to water plants in pots when they are deciduous?
I don't understand what you mean. All plants require water. Most plants grown in pots are deciduous. Are you talking about plants that are dormant?
Yes. After a plant loses it's leaves in the autumn and goes dormant. Does it still need water then?
It depends on the plant. Most houseplants don't go completely dormant and will need some water to survive. Plants that do lose all their leaves and appear to be dead, like tuberous begonias, amaryllis bulbs and some bonsai, also require a chill period and specialized care. I suggest you post your question again in this forum or the houseplant forum, asking about plants that go dormant and giving specific plant names.
I water my fruit trees about once a month in winter when they have no leaves. I think this is about the minimum. The same trees in summer need water once a day as the pots are only 5-15 gallon.