“KWin’s “Active screen follows mouse” setting is now gone; now the active screen is always the one with the cursor on it, or the last one that was tapped with a touchscreen. This turns out to be much simpler and it’s what we think most people wanted anyway, hopefully alleviating complaints about OSDs and new windows opening on unexpected screens”
That was one of my most annoying issues, but learned to work around it. But on the rare occasion where I end up clicking and I don’t realize that my mouse was on the the other screen, this will be a huge improvement. (especially when I have my Always on top windows for my job)