The aggregation system will no longer rely on the notion of primary shard for shard targeting. This includes, but is not limited to, specifying the primary shard as a merger for sharded aggregates. As such, the documentation should be updated to account for this (one such example: https://www.mongodb.com/docs/manual/core/sharded-cluster-query-router/#routing-and-results-process)

Description of Linked Ticket

• Adapt $facet’s constraints() logic to no longer depend on kPrimaryShard.
• Also delete DispatchShardPipelineResults::needsPrimaryShardMerge.