This coming Friday, January 27th, we will be releasing version nb154 of MIT App Inventor.
This is a component release, which means that it updates the component software used on devices by MIT App Inventor developed applications. Most of the changes are bugfixes or updates needed to make MIT App Inventor compatible with the latest version of Android.
By default this new property is not set. However at a future date new projects will be created with it set to true (existing projects will not be affected).
Because this is a component release, there will be a new MIT AI2 Companion released at the same time. The current Companion (version 2.39) will continue to work, but you will need the newer version (currently planned as 2.40) in order to use the ShowListsAsJson.
The release is scheduled for January 27th at 10PM US Eastern Standard Time. That is 2017-01-28T03:00:00Z (aka 3AM on Saturday, January 28th, UTC).