The new logout design doesn't want to have the vignetting. Thus the
logout effect itself doesn't make any sense any more. All that would
still be used is the logout blur which can also be provided by the
blur effect nowadays for fullscreen windows. As the new logout is a
fullscreen window it should use that one.

The logout effect did one more thing: it kept the vignetting and the
blur once the user selected logout. Now without the vignetting this
would be weird and again doesn't make much sense any more.

So overall I think it's better to just drop the logout effect and use
blur effect in normal way. Neat side advantage: it will also work on
Wayland out-of-the-box.