The way session cache is maintained, idle sessions keep accumulating in the session cache. If the workload doesn't use all the idle sessions, the oldest sessions stay open forever. In some cases these sessions might hold some resources inside WiredTiger, which can cause problems. eg: dhandles that never close in WiredTiger.
This ticket is to build a mechanism around the session cache, to cleanup old sessions that have been idle for too long.
More details in the linked tickets.