I discovered today Wobbly Windows had stopped working so I opened System Settings->Workspace Behaviour->Desktop Effects and could not find it there. However, if I search System Settings for the effect I am directed to Desktop Effects, but I still can't find it. What am I doing wrong?

On the right side of the search bar in the Desktop Effects section, there is a filter button. You might need to uncheck one or both options there if Plasma has (mis)detected your graphics capabilities, or Plasmashell has crashed at some point, and hidden it.