Honestly don’t know what happened to it because it still works on some other distro’s? Last time i checked anyway!
It seems nobody knows what happened, not even the KDE devs.
J.
Are you trying to justify my staying on XFCE (where it works?)
As GTK hater, I am not trying to justify anyone using whatever GTK based desktop environment.
J.
I guess we all have our different ideas of where the ‘tipping’ point is on changes being successful (or even improvements). I won’t worry too much about the future until it arrives 
As a long time GTK2 user, I have seen the future arrive in form of GTK3. And that future is nothing I want to participate in.
J.
But we’re moving massively off topic now.
I’m afraid that’s my specialty 
Personally still using GTK2 capabilities, as far as I know…
Shooting in the dark here, but have you tried another mouse? I’ve had various mice and trackwheels misbehave over the years and had to replace them. I know it’s a long shot but sometimes it’s hardware playing up, not just software. Forgive me if you’ve (probably) checked this. 
I’m like you also. Can’t figure it out. It used to be setup that way and then all of a sudden it didn’t work anymore. New installs …it’s not working. But like i say it does as far as i know on some other distro’s but haven’t tried them in a while.
Ah yes, the slight downside to all of the customization. I do love what KDE does though. To have all of that customization and still be arguably the lightest of the big 3 DE’s is utterly impressive. Sometimes all of that customization breaks things though. It’ll be fixed soon I’m sure.
Define soon, mousewheel based workspace switching has stopped working in April.
J.
Has anyone reported this to KDE devs?
Dunno. I use Cinnamon now because I like everything working more than customization.