The 6 month process to withdraw the use of 1 Day Travelcards on Transport for London services has now begun. Technically they would remain valid on National Rail services in the London Zones, but that seems to be an unlikely solution.
It is important to emphasize, as stated in the article attached, that this is not a final decision and the process may yet be reversed. However it seems that will only happen if TfL gets additional funding from central government.
https://haveyoursay.tfl.gov.uk/travelcards
Weekly and longer period travelcards will remain. If this does happen then Oyster and the various forms of Contactless and mobile payments, with their daily and weekly caps (and cash payments for individual tickets) will become the methods of payment by the day.