I want to know the answer to the following question:
When did improvisation begin to lessened strongly?
All the background I'm provided with is that "In opera, the practice of singing strictly by the score, i.e. come scritto, was famously propagated by soprano Maria Callas, who called this practice 'straitjacketing' and implied that it allows the intention of the composer to be understood better, especially during studying the music for the first time.". Is that enough to answer the question?
Answer: no