His slider has very little horizontal movement this year. In the last game he was getting on average just 3 inches of horizontal break on it. I'm not sure you can even call it a slider when it moves that little horizontally. It's honestly more of a cutter.
He had way better movement on his slider last year. They might have tried tinkering with his slider this year cause they were suspicious that he was tipping the pitch. And it looks like he was. There was a 2 inch difference between his average slider release point vs. fastball release point. Most guys on the Sox have about a 1 inch difference between their release points for different pitches, some even less. Liam has almost no difference between his slider and fastball release point, which is probably how he gets so many guys to swing at his sliders way out of the zone.
The problem with the tinkering is that there is still about the same difference in release point as last year, but now the shape of the pitch is worse.
It's crazy to me that Katz has gotten Cease to throw what is probably the best slider in the Majors this year, but Kopech's slider has heavily regressed.