To play them via the browser, there's a toggle button in the Details section for "Remastered aspect ratio" and if you untoggle that, it plays the episodes in 4:3.
Sky West
When I downloaded in 2022 using StreamFab, I visited the D+ web site, used that toggle on the details tab and set it to DISABLED/OFF.
The downloads from StreamFab are 1280x720 files, BUT the image is 4:3. (The black pillar bars are hard coded in the files).
I just tried to download again, with 6.1.7.0, but it doesn't even kick off the analyzing for some reason. I thought D+ was ok in the newer SF versions??
EDIT: that was odd, I had to pick a movie which started analyzing, then went back to Simpsons and it analyzed and gave me download selection list, I was able to add and start downloading.
Now with the leaked keys, its giving me 1080p (1920x1080) files, but the image inside is still 4:3. again black bars are burn into the file.