No more transaction packets, and no Action Ids. Also, the window click packet has a changed slots parameter and a clicked item parameter. The changed slots parameter is what the slots look like after they have changed. The clicked item is what's on your cursor after the click is done.
StateId is a variable in the client that should be replaced with the stateid from a set slot packet or window items packet every time one is received, then should be sent as part of the window click packet. StateId is only used in 1.17.1+.
Pre 1.17 versions sent set slot packets after closing the window to update the window, this is no longer done. You have to update your local inventory instance with what you do while you are in the inventory.