The auth_all_servers parameter controls whether to read the authentication data from one server, usually the primary node of the cluster, or to read it from all servers and form a union of the data. In practice this is only required by the schemarouter as it's the only module capable of dealing with disjoint sets of user accounts.

In MaxScale 2.5 the parameter was automatically enabled for the service whenever a schemarouter instance was used. This was changed in MaxScale 6 to require it to be explicitly configured. While the implementation in version 2.5 was very much not ideal, the behavior for the end-user was better than that of version 6. To make it easier to use the schemarouter, the functionality should be enabled automatically whenever a service uses the schemarouter module.