This was done as part of the work on RoutingKeyProviders. We now extract a routing key from an entity, and how that routing key is transformed to a shard identifier is up to the backend.

Currently the backends in Search 6 do not offer dynamic sharding; only static sharding is available, i.e. the list of shards defined on bootstrap. But theoretically they could keep a shared state listing all shards, somewhere, and rely on that state plus the routing key to route requests to shards. My point is: users no longer have to care about this in the component responsible for extracting the routing key.