Your model of 40 years ago that fluttered had very weak servos compared to today's standard. You probably also used 2-56 rods. I have seen lots of models flutter, including foamy park flyers. Every time it was apparent the linkages were weak/sloppy and the servos were not strong enough.
Full size aircraft textbooks almost always consider stick-free scenarios where the system needs to have a sort of positive stability (mass balance) to prevent flutter. Models are different because the 'stick' is fixed, based on the strength of the servo and linkages.