Previously, when a change stream was opened on mongos with the flag showMigrationEvents: true, the operation would fail on mongos with error 31123 as before.

This change lifts that restriction. Change streams with the showMigrationEvents: true flag can now be opened on mongos as well.

Note: the showMigrationEvents flag for change streams is an internal option, intended for downstream tool use.

Description of Linked Ticket

SERVER-88167 exposed additional fromMigrate: true fields to change stream events that originate from a migration. These fields are only exposed if the change stream is opened with showMigrationEvents: true.

SERVER-88167 (intentionally) kept the existing limitation of showMigrationEvents only being usable when opening a change stream on a shard mongod. That means even after SERVER-88167 trying to open a change stream on mongos would still fail with error 31123 as before.

To lift this restriction and allow opening change streams with the showMigrationEvents flag also on mongos, this erroring needs to be removed and additional sharded cluster change streams for showMigrationEvents: true need to be added.