You should check with Blue Ox. They have modified the base plates to bolt to the original bumper and bolt to the frame. This eliminates any problem with cracks in the Trailhawk frame as the bumper bracket provides reinforcement for lateral force when towing. I would bet the base plate will fit a 2019. I just finished installing blue pates on my 2018 Trailhawk. BX1136 is the part number for Trailhawks.
I don't think Jeep has gone back to the 2014 set up but a Easy way To verify if you need a flat tow harness: Look for a power steering hydraulic fluid reservoir. My 2014 has one and I don't need a towing harness cause the system is electric/hydraulic hybrid. The hydraulic fluid provides the resistance needed to keep the Jeep from doing the death wobble.
Mike and Debbie: USAF Msgt Ret, DoD Civ Ret
DAV and Good Sam Lifetime Members
2014 Allegro Open Road 36LA/2014 Jeep Grand Cherokee/2018 Cherokee Trailhawk