Bear in mind that not every change is the responsibility of the android dev team. They provide the base operating system, then manufacturers adapt it to their devices, making changes as they go. Sometimes these changes are modest (e.g. Nokia devices), sometimes they are extensive (e.g. Samsung), but it has the unfortunate consequence that if something changes you can often not be sure whether it was Google or the manufacturer who chose to make that change.
To be honest I don't recall ever seeing this setting in the first place, but then my most recent device with an SD slot is a Samsung tablet running 5.1, and as noted Samsung very much do their own thing so the absence of the setting there tells me very little.