The rewrite topic clearly had some interest so I decided to look into PLAIDNET more. It turns out I misremembered and it lived in production cars since 2020.8 to 2020.28. The nets themselves appear to be a replacement for hydranets but being more capable. In particular they could tell if a car object had turn signals or brake lights on, could tell blocked lanes/splits, HOV lanes (and if blocked), ... Also the 3D objects net is separate from plaidnest/hydranets. Also in FSD Beta a bunch of nets were renamed from CITY_STREETS to STATIC it seems.
Overall I am trying to bring in the big guns to do some actual detailed analysis. It'll probably take a while, but I'll link to it once (if) ready.