I believe that the Travelcard agreement is where the revenue from Travelcards is split between the train operating companies and TfL. By withdrawing from the agreement it would mean that Travelcards won’t be valid on TfL services but it opens up the opportunity to introduce a new zonal ticket valid exclusively on TfL services but not national rail which means that TfL would get all the profit from selling that ticket instead of sharing it with the TOCs.
In the 80s there were two tickets, The Travelcard which was valid on just London Transport services and the Capitalcard which had similar validity to today’s Travelcard. Eventually the two were merged and the Capitalcard is no more.
In other parts of the country for example Birmingham have differing tickets depending on which of the three modes of transport you want to use and this is what London may well revert back to.