The feature wasn't removed from Imaging Engine, it just hasn't made it's way into a release as of yet.
With Imaging Engine, the concept was to get a product out as soon as possible to the user base to better handle ripping of more complex files. So, as soon as Imaging Engine had enough base features, it was released.
The missing items are then being added in in subsequent releases.
Ink Selection via Public Parameters is coming in 16.
For the time being, you can use Printing Methods to do ink selection (ie make a Printing Method called "DoNotPrint", then assign that to inks that you don't want output via Public Parameter in Optimize PDF Separations, and use Selection Criteria in the Imaging Engine tasks to not rip inks that have a Printing Method of "DoNotPrint").