InnoDB Cluster Read-Only Lag Behind Writer
SHOW SLAVE STATUS is only applicable to master/slave replication channels. 

For Group Replication, you would look at the Replication_group_member_stats table. In particular the two queue sizes, the certification queue and the applier queue. The applier queue length tells you how many transactions behind this node is in applying the transactions which the group has agreed/promised to apply (in the same order) through the consensus protocol. 

That column is new in MySQL 8. You can see how I calculated it by hand here in 5.7 (the gr_applier_queue_length() function).

We also plan to add the applier queue length (lag indicator) to the status output in the MySQL Shell AdminAPI.