Post #1489: 
draguu:
i cant able to add use_simplify in pie menu in 2.8
Check this page Adding widgets
Looks like the first method doesn’t work in 2.8 yet. But you can use other methods.
And of course you can use Custom tab to write the code for the widget:
L.prop(C.scene.render, "use_simplify")
Or Property tab:
C.scene.render.use_simplify
In most cases you can find the path for the widget in python tooltips.