There are a number of new official USAPA/IFP rules for pickleball. The most impactful have to do with the serve. Timing of the the serve is much more closely defined. A whole new type of serve is allowed. And “let” serves that touch the net now must be played – the “let” has been removed.
Copies of the change document for 2021 and two different versions of the rules can be downloaded from here.