The pg_wal directory on a standby server does not get auto-purged
The replication slot should not exist on the standby, only on the master.  Unless you are using cascading replication, which you don't seem to be.

If you have a replication slot on the standby but no one is connecting to it to read from and advance it, that explains the retention.

See https://www.postgresql.org/docs/9.6/continuous-archiving.html#BACKUP-BASE-BACKUP:


  It is often a good idea to also omit from the backup the files within
  the cluster's pg_replslot/ directory, so that replication slots that
  exist on the master do not become part of the backup. Otherwise, the
  subsequent use of the backup to create a standby may result in
  indefinite retention of WAL files on the standby