Even after 2020 ADS-B out won't be required in much of the airspace in the US. Only within 30 nautical miles of a class B airport, within the lateral limits of a class C airport, within class D airspace, and above 10k feet (but not below 2500 AGL I think). Even then, its only required for aircraft originally equipped with an alternator and electrical system (except in class B).
Basically, it's optional in most uncontrolled airspace and for older aircraft.