SUMMARY The option to change the scroll speed doesn't appear on X11 using the libinput driver but it does on Wayland and on X11 using the evdev driver. SOFTWARE/OS VERSIONS KDE Plasma Version: 5.23.4 KDE Frameworks Version: 5.88.0 Qt Version: 5.15.2
That's because there is no such option in Libinput. :) On Wayland we manufacture it ourselves in KWin to work around the lack of this setting in Libinput itself. But that's not an option on X11 unfortunately.