Tesla is now recommending different charge levels depending on the vehicle and battery chemistry. It's not clear yet how many vehicles this affects (going from 90% down to 80%), but it's a large percentage.
Tesla's battery warranty is based on the vehicle holding a 70% or higher charge after about 100k miles or 8 years (exact mileage and age varies depending on model). It's likely that Tesla realized that charging to 80% instead of 90% did in fact extend battery life and therefore reduced warranty claims, but Tesla hasn't made an announcement on why this change has taken place besides this being their updated "recommendation."