If I remember correctly the youtube channel The Swords Path tried to somewhat recreate the witcher fencing style. You can check some of their older vids.
The issue is, the witcher fencing techniques would not really work that great. There is no realistic fencing school that would resemble it. Even in the books, the style is developed not to fence with humans - it's to fight monsters. It's what witchers do, so it's what they learn. It works for witchers because they are inhumanly quick.
In real life fencing, you don't really want to do pirouettes and expose your back to your enemy. There might be a situation where you could get away with it, but not consistently, at least not with longsword or similar weapons. There are weapons, like a zweihander where you do spin around but that's because you have a bigass sword which would have longer reach and you would really only use it to keep enemies away from you.
The witcher style looks good on page or screen, but it is not how a real person would fight with a sword. Look up some HEMA (historical european martial arts) sparring videos to see swordfighting based on historical sources. You probably mostly want to see longsword fights.
If you have questions, feel free to ask. I'm a bit rusty as I'm on a bit of a hiatus from HEMA but I can try answering for sure.