Note that this ticket is to discuss about the shard-id field that was introduced on the recipient coordinator document as part of SERVER-60645.

This field is there just for debugging purposes + being able to reuse some utils such as the ActiveMigrationRegistry. The underlying problem with the shard ids is that they are not  portable when we do an snapshot of a sharded cluster and restore it in another cluster. In this situation we have to run some scripts to properly update the metadata.