Yup - the standard was published in like 2008 but since it was voluntary it took a while for companies to adopt it. Toyota was the first around 2011, but Ford/GM/Ram all played chicken with who would go first. Ford pulled the trigger and announced their 2015 model year adoption (and the other two followed suit).
Also there have been so many advancements in vehicles and technology and expectations, so today’s 1/2 ton baseline braking/axles/etc. could rival yesteryear’s HD models, which further complicates comparisons.
So while SAE J2807 might not be the best yardstick for pulling a horse trailer in terms of the testing parameters, if you look at the ratings post 2015 you can at least compare across the board “as equipped.”