Design reset rules have been in place since July 2018...
New Design Balancing system when changing leagues
Before this there were huge problems. Promoted teams started the season with a design equivalent to their previous tier (almost inevitable they would be relegated again at the end of the season). Relegated teams would start the season with overpowered design for the tier. We also had a second season bug for people joining a different league. IIRC this was much more pronounced than the existing N+1 bug and it was impossible to reset a car to have the same design as the rest of the league.