Theoretically all files should be un-moddable, but obviously people have found ways around that in the past. However, the last 2 years they seemed to have locked it down much better than before.
Because of people modifying cars performance and cheating on line Code Masters have made this release unmoddable apart from the helmets. You were able until this release to add induvidual trackside banners and advertisements for each track which made the sim more immersible for me and many others. Thank you to those people who cheat.