I have depooled a replica in es5 to do maintenance and after an hour, there are still connections to it from the entity data dumper. That's the only maint script/dumper that still holds and keeps connecting to the depooled db after this time. This problem hinders DBAs ability to do maintenance (and more importantly primary switchovers are not getting reflected meaning the primary can get a lot of reads).
This is similar to T298485: MW scripts should reload the database config and a proper fix would be to reload config on the fly but at least (and as a quick measure), the dumper should not take this long, it should be split to smaller batches that take maybe an hour or less.