Yes, this is true. If you book a placeholder (that is, you put down a $250 deposit to be applied to a specific cruise later) you can get a refund at any time, and if you don't either book a cruise or request a refund after 24 months, you will receive a refund to your original form of payment.A reputable website says yes, if you decide to not book a room with you onboard booking deposit, you can get your deposit back before the 24 month window ends. If you haven’t sailed before the 24 months, your money will be refunded.
If you book a specific cruise, you can also cancel and get a full refund up until a few months before the cruise (currently 90 days for cruises of 5 nights or fewer, 120 days for cruises of 6 days or more). An exception is concierge booking, for which the deposit is non-refundable. The exact details for cancellation fees will be provided when you book.
You can also move a cruise without penalty. You will, however, lose the 10% onboard booking discount if the new cruise sets sail more than 24 months after your onboard booking. If you do move a cruise booked onboard, make sure to say you are "moving a reservation" and not "cancelling and rebooking" -- you will lose the onboard discount if you cancel the original cruise.