In 4.16 P1 and P2 my C++ Blueprintable Interface no longer shows up in the “Add Interface” menu in Class Settings.
This interface is visible and selectable in all engine versions from 4.12 - 4.15.2, however it is not in the current 4.16 preview builds.
In fact the ONLY interfaces currently selectable are all blueprint created interfaces. This means that all C++ derived ENGINE interfaces are also missing (TurnBasedMatchInterface, AISightTargetInterface, BlendableInterface, ect).
Unless there was an as of yet unannounced change in behavior in 4.16 regarding interfaces this should be a bug.