When sanitization for EventLogging events was added, a job to drop old unsanitized data was added in https://phabricator.wikimedia.org/T209503#4991424. At that time, it looks like the WMDEBanner* tables were somehow excluded from having their data dropped.
T273789: Sanitize and ingest all event tables into the event_sanitized database is refactoring and formalizing sanitization and data purging for all tables in the Hive event database. Can we drop the old data from WMDEBanner* tables? Does it need to be sanitized and kept indefinitely in the event_sanitized database?