I'm guessing there'll be some sort of update to the Disney+ apps and website to allow switching between the 4:3 and 16:9 versions of The Simpsons? Otherwise they'll have to have each version listed separately which could be confusing.
I'm hoping they'll upload the 4:3 versions of the other shows they only have in cropped 16:9 too at the same time, but I won't get my hopes up there.
Why would they need it to switch?
Last time I saw something 4:3 on Disney+, it was pillarboxed. Don't see why they couldn't pillarbox 4:3 Simpsons, okay its not ideal for non widescreen TVs but they must be in the absolute minority by now?
I've noticed an "app settings" option has appeared which wasn't there before (both through web and Roku, can't speak for other platforms). Not sure when it appeared, but it wasn't there a couple of weeks ago. Only has options for streaming quality right now (three options- Automatic/4K, Moderate/HD and "save data"/SD), but I presume that's where the 4:3/16:9 toggle will be if there's going to be one.