global setting "substitute_variables" is rejected as unknown global parameter

I'm trying to configure the MaxScale proxy and would like to use environment variable substitution in the maxscale.cnf file.
I've prepared a custon maxscale.cnf which starts as follows:

[MaxScale]
substitute_variables=true
threads=auto
...

(I've also tried substitute_variables=1 without success)

When starting up (maxscale -d -l stdout) I get the following error message:

Info : MaxScale will be run in the terminal process.
2019-03-18 21:52:08 notice : syslog logging is enabled.
2019-03-18 21:52:08 notice : maxlog logging is enabled.
2019-03-18 21:52:08 error : Unknown global parameter 'substitute_variables'.
2019-03-18 21:52:08 error : Failed to parse configuration file /etc/maxscale.cnf. Error on line 2.

When searching the web, I could only find one other reference to this error: Error mentionned in KB