If run_background_tasks_on is set to anything other than master, Synapse will try to run background jobs on a worker, including the message retention policies support's purge jobs. However, these jobs end up calling the purge_history function of the storage layer (in the PurgeEventsStorage class) which doesn't exist on a worker, therefore the jobs fail with AttributeError: 'GenericWorkerSlavedStore' object has no attribute 'purge_history'.

This bug was introduced in #8513

cc @iansinclair