As far as I know the just-in-time sets are still available and haven't really changed since 2016.
In the past there were different versions of the Homework Sets Editor, which is why some older instructions may refer to things like Homework Sets Editor 2. They have since been merged into a single Homework Sets Editor. When you click on the "Edit Problems" link for an individual set, you can set the Assignment Time to "just-in-time". After you have clicked "save changes" you will then be presented with the options for just in time sets.