I've created this thread so all the publishers using link units that have experienced a drop in revenue can confirm that they are having the same issue.
I have contacted a GCPP but I have no answer from them yet.
I invite you to check your account and share here if you are getting this "Unbillable clicks". These are the steps you have to follow to find it out.
1- Left column, click on "Reports" and then in "Entire account by day"
2- In the top right corner, select a date range, starting from 1 January 2018 to now.
3 - In the main content area, over the input box, select "Report by" : "Months" : "Bid types"
4 - Look for "Show rows" and select 100 or more.
5 - Click twice on the "Bid types" header in the results area.
Then you'll see all the unbillable impressions and clicks ordered by months. I would need you to check the clicks column. There if you are affected by this issue you will see that before August 2018 there are 0 unbillable clicks, and after that month Unbilable clicks start to grow and the revenue is almost halved.