How (not) to manage SSRS report-execution history retention
Interesting find.  I was unaware of this setting until I built a page that used the SSRS execution history table as the sole source of a report's download count.  To my surprise, the counts disappeared one day.  At that point, I discovered the default value of 60 days retention.  I recall increasing the retention setting in the GUI, but I did not set to 0.  

I re-engineered my download count to be stored in a custom table and updated daily via a SQL Agent job using the SSRS history, in order to eliminate any other surprises.  There is definitely conflicting documentation on what "0" accomplishes.  However, if you set the retention days to a very large number (several hundred years), then that is essentially an infinite retention period and should accomplish your goal.