There were no more Trials of the Grasses because there's no one to perform/endure the Trial any more:
The general public considered witchers monsters, and did not want their children to become witchers, which considerably lowered the number of candidates available for the schools to perform the Trial on.
Only 3 in 10 survived the Trial, severely reducing the number of of people who became witchers, who would collect more children for their schools and/or support the schools.
The recipe was kept secret, and was lost when Kaer Morhen was attacked. Since the games are not canon, they were able to get the recipe through Yennifer.
With few people joining, even fewer surviving to the end, the dangerous job, society making life generally difficult, and the secrecy around the recipe, there were few people to support the schools and/or knew how to perform the Trial completely. This made it easy to lose the recipe, which it was, when Kaer Morhen was attacked.
If I remember correctly from The Witcher 3, Ciri was the last person to train as a witcher at Kaer Morhen, but never went through the Trials.