Google quietly announced on Google+ that they are dropping the ability to see the reach and frequency reports from within the dimensions tab in your AdWords reporting console.
Starting January 28th those two reports will only be available within the Campaigns tab.
In addition, API access to the report in the 'Dimensions' tab will no longer be available after the AdWords API V201406 is sunset in April 2015.
But Google said they improved the two reports within the campaigns tab by showing aggregate results for any time range, not just individual days or weeks, and you can see reach and frequency metrics for any Display campaign.
Forum discussion at Google+.
