How can I add airline tickets to Apple Wallet?

You will be able to add airline tickets to Apple Wallet any time your ticket has been released with an "Add to Apple Wallet" symbol. This may differentiate based on airline. Contact your airline for more information.

Some additional information about boarding passes in Apple Wallet can be found here: Add, use, and share boarding passes, tickets, and other passes in Apple Wallet - Apple Support

I suppose it depends on what you mean by “ticket”. Only boarding passes can be added to your Apple Wallet and boarding passes are issued electronically or physically when you check in online or in person. So yes, many airlines only let you check in 24 hours before boarding time. We fly Southwest and American. Once we have our boarding passes (electronic, not paper of course) there’s always an option to add them to our Apple Wallet and Apple Watch. I personally use my Apple Watch to board a plane these days.