Patch notes for all updates should be mandatory.
If I purchase a product and the developer is changing anything about that product, I think there should be very clear transparency as to what is being changed.
Whether it's a small patch of only a couple megabytes in size or a large patch that completely causes you to have to re-download the game, it doesn't matter. If they're changing something, we should know.
There have been many times that a game has released a patch without there being any notes on what that patch did whatsoever. I understand that most patches are to it improve the game and the experience playing it, but it's also a perfect opportunity for companies to negatively change things as well. if they can't provide the patch notes then the update should not be allowed to be released.