check slave status innodb cluster
MySQL InnoDB cluster relies on Group Replication, which provides the mechanism to replicate data within InnoDB clusters with built-in failover, and not on the asynchronous replication. However, whenever you join an instance to a cluster the distributed recovery process kicks-in, and this process uses the asynchronous master-slave replication until the instance finishes joining the group.

As stated on the manual: "Group Replication distributed recovery can be summarized as the process through which a server gets missing transactions from the group so that it can then join the group having processed the same set of transactions as the other group members. During distributed recovery, the server joining the group buffers any transactions and membership events that happen while the server joining the group is receiving the transactions required from the group. Once the server joining the group has received all of the group's transactions, it applies the transactions that were buffered during the recovery process. At the end of this process the server then joins the group as an online member.".

Concluding, as soon as the distributed recovery process is complete you can (if you really need) delete the bin logs on the primary safely.

For further information, please take a look a the InnoDB cluster userguide.

Cheers,

Miguel