What do you think? Are the days of truly flexible adventure/encounter design over?
Given the PCs goals, there will be optimal and sub-optimal approaches to every encounter. A competent writer or GM is going to be pretty good at guessing the likely choices.
I don't think encounters were ever really written to be "truly flexible" - stats and information have always been given for the most likely choices, and the rest has been left up to the GM to improvise.