Today, on YouTube\'s revision history page it is stated that
The channel resource\'s contentDetails.relatedPlaylists.watchHistory and contentDetails.rel
The changelog confirms that the watch history and watch later playlists return empty lists. Unfortunately, no alternative is mentioned.
September 15, 2016.
In addition, requests to retrieve playlist details (playlists.list) or playlist items (playlistItems.list) for a channel's watch history or watch later playlist now return empty lists. This behavior is true for the new values, HL and WL, as well as for any watch history or watch later playlist IDs that your API Client may have already stored.
This data will not be available via the YouTube API service after September 15, 2016.