They switched to MAUs because it's a better metric for almost all ActiBlizz games.
Mobile games don't have a subscription.
CoD games don't have a subscription (except the yearly new release).
D3/Overwatch/WC3/HoTS don't have a subscription.
So it makes everything a lot easier to see in similar terms. Number of WoW Subs should be very similar to MAUs for WoW specifically, unless people stay subbed but don't log in.