Really? Let’s examine that statement.
FH4 was released early Oct 2018. So let’s look at some Championships from that time: Summer Forzathon Shop, events, and cars through November 1
Nope, one requires the Datsun 510, the other requires a Retro Super Car.
Maybe it changed later? How about 2019? Summer #Forzathon Shop, Festival Playlist events and rewards through July 11
Nope. B700 Front-engine, FWD; S1 Country = Italy; and S1 Super GT
Maybe even later? 2020: Summer #Forzathon Shop, Festival Playlist events and Rewards through June 11
Still no. (D500) 1950s models; (S1 900) Modern Supercars; and (A800) Vans and Utility.
2021: Summer #Forzathon Shop, Festival Playlist events and Rewards through June 10
Surprise, still no. (S2) French cars; (A800) Super Saloons; and (A800) Sports Utility Vehicles
In this random sample, not a single Championship the 599xx could qualify for. Unless you can provide something to counter this documentation, you’re misremembering.
Edit: just read your post again. PR Championship? What in the world is that supposed to mean?