Is degradation a kind of sadism?

They're separate. Sadism is about physical sensation; degradation is about language. One leaves marks, the other doesn't, and the two need completely different negotiation.

Physical limits are relatively easy to describe and you find the edges gradually, with feedback the whole way. Words don't work like that: one specific phrase can land as thrilling and the next can genuinely wound, and there is no gentle approach that finds the line safely.

Anyone doing the verbal kind should be writing down the exact words that are in and out before anything starts. Anyone who thinks the two are interchangeable is going to hurt someone by accident.