Off topic, and related to Lyc bringing up subdivisions of fantasy. Let's begin by saying most labels and genres are subjective, unless they actually describe or characterize an entity based on structural, functional, or other features (wow OT inside an OT!).
Anyway, I think we all agree that high fantasy usually involves, among other elements, a clear distinction between roles: good vs. evil, law vs. chaos. It is usually presented in such a manner that often heroes/villains arise and they battle each other. A good example is of course The Lord of the Rings and Dungeons & Dragons.
Low fantasy was, I used to believe, the opposite, with blurred morality lines, gritty themes and a diversity of characters who show complementary traits, if not contradicting. Here we could probably think of both white wolves, Elric of Melnibone and Geralt of Rivia.
However, I recall reading somewhere that low vs. high fantasy has to do more with the structure of the world. In high fantasy settings, the fantasy world is all that exists and it is the ultimate reality. In low fantasy universes, they are either related to our "real" world or they have a way of accessing it, perhaps through portals or doors or pathways across an
axis mundi a-la Yggdrasil. A very obvious example is, of course, Alice in Wonderland.
In The Witcher universe, monsters supposedly appeared after the conjunction of the spheres, and according to elves so did humans. The Wild Hunt appears to be a cavalcade of plane traveling specters. Could this mean there are indeed other worlds, perhaps like our own, and that there is perhaps a way to move between dimensions? This would explain the anachronisms, like Triss talking about evolution.
Anyway, I suppose no matter the perspective The Witcher is low fantasy. Just thinking out loud