Seem to be a number of works available on the topic, correct? Is the question specifically "what happened to the master's disputation?" Probably the 20th c. practice of accumulating defenses for several theses and having that count as your credential was replaced by the PhD as an early-career credential for European researchers instead of a "they only find it next to you when you die" type of summative work.
Prob. best to just find a Dutch PhD supervisor though