Starting 
September 10, 2018, AdWords API and AdWords scripts reports will start returning no values (
two dashes) for the following 
assisted-conversions fields for 
all API versions:
Why are we deprecating these reporting fields?
Often, the last click before a conversion gets all the credit. But along the way, other clicks and impressions might have guided your customers toward that conversion. Previously, assisted-conversions reporting fields were created to give conversions to the clicks and impressions that assisted in such a scenario. However, it’s still not easy to compare those conversion values between campaigns, ad groups, and keywords, as conversion metrics are double-counted and not normalized.
With the advent of 
attribution models that allow you to assign fractional credits to multiple clicks that contribute to conversions, you can now distribute credits among many clicks in a way that they can be summed up to 1.00. Conversion reporting fields, such as 
Conversions, 
AllConversions, and 
CrossDeviceConversions, are now returned based on the fractional-credit model, so please migrate to those reporting fields instead.
As always, if you have any questions or concerns, please post on our 
forum.
 - Thanet Praneenararat, Google Ads API Team
 - Thanet Praneenararat, Google Ads API Team