The complete patch notes for the Endwalker expansion for Final Fantasy XIV has been released. You can find the complete article here. Most of the important and major changes have been released in the preliminary patch notes, and you can find a brief summary here.
All Final Fantasy XIV worlds are currently down for maintenance. If you would like to login to the game as soon as it comes up, it may be a good idea to open the game client to see if new downloads are available so that you can start downloading the expansion prior to the servers starting again. There is a countdown for when the servers are expected to go online again below.