On behalf of the many of us experiencing this super annoying problem, official acknowledgement of this it by Xfinity would be greatly appreciated.
A few months ago (perhaps around May?), SD programs on the X1 Netflix app started playing SD shows as 16:9 stretch instead of 4:3 pillarbox. I have tried everything people have suggested on forums, but I can’t make it stop doing it. It happens on all 4 TVs in my house across three different X1 boxes, so it is not the TVs. This has to be an Xfinity issue, as it was all working fine and then on day broke on every box.
To experience the problem yourself on an X1 system, watch S1:E1 of Star Trek The Next Generation.
there are other threads on this (search?) and Netflix support replied that they do not support SD for the APP. The issue seems to be that the app does not 'respect' the original 4:3 aspect nor display it in something called AFD format. net effect is you will see it stretched on the X1 app. pehaps you could just cast to your TV or use the smart tv netflix app or a roku etc netflix app.