Isn't the historical progression cybernetics turns into control theory (via Norbert Weiner) and control theory turns into reinforcement learning. So just had a few rebrandings and never went away.
There was a particular schelling point in the '40s surrounding Wiener (and Shannon, Von Neumann, etc) and the Macy Conferences who were searching for an interdisciplinary umbrella, and found it in Cybernetics. It began as something decidedly "para-academic" and never really grew beyond that. The scope of Cybernetics was — and probably is — too broad to fit into a rigid departmental system. Instead of thinking of it as "going" away, it is probably more accurate to say that it never really arrived.