ShardingMetadata is for mongos to report GLE but in SERVER-57390, we could not remove it all together because of sharded cluster upgrade/downgrade scenarios.

As soon as FCV 5.1 is defined, we should stop calling writeToMetadata under FCV 5.1 or greater so that eventually we can remove ShardingMetadata after 6.0 is released.

For the code and discussion, see this discussion thread.

https://github.com/10gen/mongo/pull/113#discussion_r674244478