The MaxScale documentation for ReadConnRoute says the following:

router_options can contain a list of valid server roles. These roles are used as the valid types of servers the router will form connections to when new sessions are created.

router_options=slave

Here is a list of all possible values for the router_options.

https://mariadb.com/kb/en/mariadb-enterprise/mariadb-maxscale-22-readconnroute/#router-options

As you can see, it says that router_options can refer to a list, but it does not say what that list should look like. I assume that it means that it should be a CSV, such as:

router_options=master,slave

If so, then the documentation should probably say so.