They do have to pay the actors if they play clips of the past, which is really too bad because there’s so much good stuff I’m sure we’d all love to see.
One exception to this is if they had it in their contract that all rights to past episodes remain the sole property of the show. I believe in that case they can do so. The only problem with that is, no agent in their right mind would ever allow a client to sign a contract with that condition in it, except maybe a brand new actor who is trying to break into the business and will make a concession to that to get the job.