The US air force still flys KC-10s and probably will for a while.
IIRC, the Netherlands do as well. Not sure though.
A while? Those KC-10s will be in the service for, more than likely, decades to come. Any tanker chosen by the USAF will take many years to get into flight testing, then many more years being approved for production. By that time, the original order will have been reduced, the price per aircraft will balloon into the hundreds of million a piece, and those KC-135s, KC-10s, and KC-130s will soldier on. I have little doubt I'll be dead before the KC-10 leaves the inventory...