In the Blood of Elves novel, Triss does mention it at the start of the book.
It’s clear, she suddenly thought, feeling a passionate arousal of an entirely different nature. It’s obvious. They want to mutate the child, subject her to the Trial of Grasses and Changes, but they don’t know how to do it. Vesemir was the
only witcher left from the previous generation, and he was only a fencing instructor. The Laboratorium, hidden in the vaults of Kaer Morhen, with its dusty demi—johns of elixirs, the alembics, ovens and retorts . . . None of the witchers knew
how to use them. The mutagenic elixirs had been concocted by some renegade wizard in the distant past and then perfected over the years by the Wizard’s successors, who had, over the years, magically controlled the process of Changes to which children were subjected. And at a vital moment the chain had snapped. There was no more magical knowledge or power. The witchers had the herbs and Grasses, they had the Laboratorium. They knew the recipe. But they had no
So we know of this that the Witchers of Kaer Morhen still had the tools to perform the trial, but Vesemir was the only one from the previous generation where it was more common. As by another account of Triss upon finding a young trainee on the the trail towards Kaer Morhen that no witchers had been trained from children for a quarter of a century.
She wanted to catch a glimpse of the little witcher once again — children
had not been trained in Kaer Morhen for near to a quarter of a century.
The main problem for the witchers was that they didn't really know how to perform it since it was usually done by Wizards, and by an account of Triss in the same chapter I think it is reasonable that the witchers were hesitant to ask for the aid of wizards.
I can understand that, thought the magician. A child trained to be a witcher, a girl, at that, who has not undergone the mutations, should not be told such things. A child like that should not hear about the massacre. A child like that should not be terrified by the prospect that they too may one day hear words describing it like those which were screamed by the fanatics who marched on Kaer Morhen long ago. Mutant. Monster. Freak. Damned by the gods, a
creature contrary to nature. No, I do not blame the witchers for not telling you about it, little Ciri. And I shan’t tell you either. I have even more reason to be silent. Because I am a wizard, and without the aid of wizards those fanatics would never have conquered the castle. And that hideous lampoon, that widely distributed Monstrum which stirred
the fanatics up and drove them to such wickedness was also, apparently, some wizard’s anonymous work. But I, little
Ciri, do not recognise collective responsibility, I do not feel the need to expiate the events which took place half a century before my birth. And the skeletons which are meant to serve as an eternal reminder will ultimately rot away completely, disintegrate into dust and be forgotten, will disappear with the wind which constantly whips the mountainside. . .
So they had all the tools to perform the trial, but no one was left with the knowledge of how to use them, and they lacked a wizard to help them do it.