In the MaxScale documentation in GitHub and the KB, there is a typo on the page for readconnroute:

max_replication_lag
Type: duration
Mandatory: No
Dynamic: Yes
Default: 0s
The maximum acceptable replication lag. The value is in seconds and is specified as documented here. This feature is disabled by default.

The replication lag of a server must be less than the configured value in order for it to be used for routing. To configure the router to not allow any lag, use max_slave_replication_lag=1.

Note that the parameter is called max_replication_lag, but there is a typo in the description that mentions max_slave_replication_lag.