The change was made by Google.
I have no insight into the Android development process.
I can’t find anything definitive either, but I can easily see how not giving away the phone’s content upon any random USB connection would improve security.
And if this was indeed the intention, then making the corresponding developer setting defunct is just consistent, since enabling the developer settings is no hurdle for anybody and given as advice all over the internet … If this setting still worked, I guess the change would be almost meaningless.
After a bit of internet searching this really is the most logical explanation.
This setting does not lead to a USB mode being used as a default persistently from Android 6 on, although mileage may still vary as phone vendors can customize their Android … but luckily they more and more refrain from doing that in favour of easy and timely security updates.
However, as long as this setting might serve as a workaround for certain problems (example), I wouldn’t hastily call for removing it.