What do you mean by affect? The 3rd party content is still available, the only change is if some 3rd party content does not have a set of icons painted to present the car skin view when selecting cars. All the S397 DLC have this, but not all 3rd party car sets. That is up to the original modder. As far as other items? If it was a workshop item or package install, those should all be just the same. Are you using non-standard folders?(i.e. Steam on drive C:\ but the rF2 stuff on an SSd?
I´ve had several tracks not working (shows on the location list but no layout actually appears) with the new ui. Probably due to the wai they packed or grouped their mas files so it´s most likely up to the modder to update that content. Besides that yeah, old car icons need to be updated and the old track icons and loading screens aren´t used anymore
