New and old designs have been tested and can work with unleaded fuel. (Edit: there are surely some exceptions here, so I don’t mean this to be all encompassing)

New and old designs have been tested and can work with unleaded fuel. (Edit: there are surely some exceptions here, so I don’t mean this to be all encompassing) New designs must work with leaded fuel since that is the fuel which is available - until avgas contains no lead this will continue to be the case. Side note, most new planes are sold without an ecu.