Unfortunately this checkbox will not be implemented anymore.
There was a request to do it here
https://github.com/bitcoin/bitcoin/issues/12520
The solution was to remove the "New" Button in the address book so it's only possible to create new addresses via the "Receive" tab.
This is the pull request which is already merged into the bitcoin master branch.
https://github.com/bitcoin/bitcoin/pull/12721
So with the next bitcoin core release there is no more "New" Button in the address book.
I already implemented this checkbox as I also missed it, but before I got the chance to create a pull request, the "New" Button was removed so my changes are useless now.
What, are you saying that you will not be able to choose between nested and bech32 format anymore? from what I understand, it reads as if they will remove the checkbox... why? How will I choose what format do I want to generate then?
In my opinion I still believe a sliding menu would be the best way:
Having this there next to the button that generates addresses is very straight forward and clear.