My family was able to book dinner at Palo while onboard the Fantasy when we realized there was a night when we wouldn't like much on the main dining room menu. Guests are restricted to booking 1 meal of each type (e.g., Palo brunch, Palo dinner, Remy brunch, Remy dinner) in advance of a cruise, but additional reservations can be made on board if there is availability.
My recollection based on reports from pre-2021 cruisers is that when Cabanas was open for dinner, the menu was selected from the main dining room menus, so it wasn't different food, just a different environment. Given that people commented on how much less crowded it was there, I am guessing that Disney Cruise decided it wasn't worth the additional cost of crew to staff it, and that it would be better to have "swing" staff in the main dining rooms.
Disney has 2 ships (Dream and Fantasy) with no live dining room shows, and 2 ships (Magic and Wonder) with 1 dining room with live shows. Those ships are available for people who want a Disney experience without as many shows as on the Wish, Treasure, and Destiny.
