I have a Dell Latitude E7240 laptop running Fedora 26 with KDE Plasma 5.10.4 and KDE Frameworks 5.36.0. When I press the Toggle Touchpad button (fn-F5 on this laptop), the touchpad is turned off but will not turn on again by pressing this shortcut. Going into System Settings > Shortcuts and rebinding the Toggle Touchpad shortcut to some other key (such as Ctrl-Shift-Alt-T) will not change this behaviour; the key won't toggle the touchpad back on no matter how many times I press it, and every time it's pressed the "Touchpad Off" message appears in the center of the screen. The touchpad can be turned back on with the Enable Touchpad shortcut as normal, although on this laptop there is no dedicated key for that so I had to rebind it to Shift-Toggle Touchpad.
*** This bug has been marked as a duplicate of bug 370588 ***