Let me guess: someone is going to spin how it makes perfect sense to persist hard buttons for volume but eliminate the hard button for mute... probably even offer an example of how important it is to be able to change volume without having to look at the screen but it's not as important to be able to mute volume without looking at a screen.
The mute is integrated into the volume buttons now. You hold the volume down button and it will mute. Personally I have my iPad volume off most of the time, so I won't miss the mute button, but I frequently used the lock screen rotation function, so I'm not sure if I'll miss the mechanical button yet. (I'm sure I'll get used to it, whatever the case.)