syncApply currently throws NamespaceNotFound exceptions on delete operations that refer to non-existent namespaces, specifically in cases when the UUID resolves to a missing collection, or when the database referred in the "ns" field does not exist. This condition does not occur during normal server operation in 3.6. However, modifications to the applyOps command proposed in
SERVER-32913 may introduce such delete operations, requested by the client, into the oplog application process on the secondary. Therefore, we should consider treating deletes on the secondary as no-ops.