Go into debug settings, set RenderResolutionDivisor to any value. Nothing happens until you relog.

Once you've relogged, the effect will start and you're able to change it as long as you don't set it to 1, at which point it stops working again.

When the effect is active, it no longer just pixelates the screen, but it seems to mess up the camera by rendering only a corner of the scene. To test, set the value to 4, relog, and lower the value one by one.

The pictures are roughly the same camera placement with RenderResolutionDivisor at 2 and 1. When set to 2, it seems to only render a corner of the screen. Prior to PBR, the feature would only lower your resolution.