With FCV > 4.2, Backups requires the backup module to be activated on at least one member per shard for sharded clusters but it also doesn't stop one from starting backups if the backup module is activated only on one member of the entire sharded cluster, for example, activating the backup module on a single member of the config server replica set and not on any member of the shards allow for one to start backups.

Currently, if backups are enabled and at least one of the Replica sets in the Cluster has a backup module, backups will start for the cluster.
The backups in the "missing" backup agent function scenario would not be consistent.