phase out

verb

1. To remove or relinquish the use of (something) little by little, either via discrete diminishing phases or (by extension) by continuous gradations; to remove in phases, or as if by phases (gradually).

   “Leaded petrol was phased out in the course of the 1990s.”

   “It [Greenpeace] also says that tax exemptions should be phased out for airlines, including the opt-outs that low-cost carriers currently have from paying VAT or fuel duty on kerosene.”