FWIW, my Yukon already has the park assist, came with the vehicle.
But...my wife got a new '21 Cadillac XT4 last winter without park assist...retrofit contract. When we set up her appt to get the heated seat retorfit done last week I asked about the park assist as well.
Just got a call back from the parts dept yesterday saying the park assist retrofit was pushed out to Q1 2023 (Q1 means the END of Q1 in my 35 years of automotive experience). So I would assume Yukons should be the same but if Caddi dealership has this info, GMC dealers should too...call your dealer and ask!