Affects all versions, but only reported on 23.08.

Galera Monitor does not need any special privileges to monitor a Galera Cluster. The monitor does require REPLICA MONITOR to detect traditional replication (with "SHOW SLAVE STATUS"). The monitor runs "SHOW SLAVE STATUS" on all monitored servers but ignores errors on executing the query, as Galera-related status information could still be queried. If the monitor lacks the REPLICA MONITOR-privilege, the only effect is the server's Access_denied_errors counter increasing.

As "SHOW SLAVE STATUS" is an extra query the Galera Monitor only uses to detect traditional replicas (not Galera replication), this usually does not affect the user. The issue was detected by a user who noticed the access denied error counter continuously increasing and was confused as to why, as MaxScale did not log an error or a warning.
To show the error counter, run SHOW GLOBAL STATUS LIKE '%Access%';