It's a good question, and reminded me of some of the functionality in 10g I still miss today.
The 10g Decision activity's closest relative (more like a cousin than a brother) in 12c is the "Simple" User activity.
As you've already figured out, the 10g Decision activity's learning capability that provided recommendations does not have a directly corresponding activity in 12c OOTB. Not exactly the same, but a potential workaround would be to use BAM to track the history and provide the end user the recommendation. The drawback to this is that OOTB it tracks all of the history of the process and not the percentage of time which path was taken from a specific activity. What you could do though would be to add counters in the process for each of the potentially targeted activities, and your process could have business indicators tracked by measurement marks for each of the "features" currently being tracked in your current Decision activity.
You could then use the process counters and business indicators to populate a BAM dashboard to help the user determine the next best action. Your simple user activity's user interface could have a link to the dashboard that displays this BAM dashboard that has the counters for the different paths taken. The same dashboard displayed could also show the dimension and measures (the "features" in the 10g Decision activity) coming from the process's business indicators in other chart(s) on the same dashboard to help the user make a decision.
I'll admit it's not a perfect solution, but as close as I can think of.
Hope this helps a little. Again, thanks for asking the question.