Grace from the Google Earth Team posted a message in the Google Maps Help forum notifying users of changes coming to Google Earth Pro and Google Earth.
On December 17th, Google updated the Google Earth TOS document and they notified customers of this change back in June 2015.
Google will stop offering their normal support for Google Earth Pro customers and instead, make them use the forum for support. Which means their questions may go unanswered.
You no longer need Earth Pro keys, you can instead use your email address and the key GEPFREE to sign in once you download the software. Those who have keys can still use them.
Google is removing U.S. Demographics, U.S. Parcels Data and U.S. Daily Traffic Counts from the data layers.
Forum discussion at Google Maps Help.