The UpdateHistoricalCounts setting controls the execution of update_historical_counts database procedure: configuring it to 0, you are going to disable the procedure.
This procedure inserts data into BFEnterprise tables HISTORICAL_FIXLET_COUNTS, for the historical information about the number of Fixlets at different severity levels, and HISTORICAL_COMPUTER_COUNTS, for the information about historical computer count objects.
The data from these tables are taken by a couple of session relevance inspectors, runnable from WebReports QNA: “all fixlet count” and “all computer count”.
For this reason, if you have any custom report based one of the two inspectors, you won’t see the report data to change and to get updated but you will keep seeing the data collected until you set the UpdateHistoricalCounts to 0.