It seems like some cards are never included in clan war in Clash Royale....For example, Zap is never there even if we get all 40 cards....Is there anyway I could find out what cards are never included in clan war?
All cards are eligible for clan war decks. If you haven't seen one appear, it is simply due to randomness.
As Alex Myers mentioned, all cards are included. Sometimes they wait a few weeks before adding new cards to the clan war pool to give the players time to level the card up. Since earthquake is fairly new and already in clan wars (my clan got it right in the current war), they probably add the cards pretty soon after release now.
There seems to be a bug. Like shown on this reddit thread the Goblin Cage is already in clan war although it's released in 18 days.