Rather than actively purge we probably want to use https://www.mediawiki.org/wiki/Manual:Hooks/RejectParserCacheValue
Monitoring of the rejection rate will be possible at https://grafana.wikimedia.org/d/000000106/parser-cache
Start Time: 23:00:00 UTC 6 April 2019
End Time: TBA (The EndTime should be once T249596: Rebuild wb_items_per_site, after incident where wb_items_per_site was dropped has been completed and the table data is restored)
The time period that we are rejecting for will be rolled out in increments to ensure that we don't have to reparse too many pages at once.
Starting with a 1 hour time period and evaluating the time period after the initial deployment.