(After reading through that issue I appreciate the R community all the more)
Unfortunately, the component was again ignored. However, many have risen to the challenge of creating an alternative and would I try Google-ing around for "bootstrap 4 multi level menu/dropdown/navbar."
Coming back around to shiny, even if a later version of Bootstrap would include these menus, Bootstrap 3, the version currently used by shiny, will not. The Bootstrap team still pushes small updates, see here for upcoming tweaks and fixes, but I cannot see them retroactively adding such a component. Alternatively someone could build a standalone menu component on top of Bootstrap 3 specifically for shiny and submit a pull request. I would check-in with the shiny team first on that one.
I hope the mini history lesson was entertaining. I am sorry I don't have a tried and true solution.
p.s. I would also welcome an issue or pull request to dull, a Bootstrap 4 version of shiny I have been working on.