See https://blogs.dropbox.com/developers/2016/06/api-v1-deprecated/
The Dropbox external storage app needs to be rewritten with the new API endpoints if we want to continue supporting Dropbox as external storage: https://github.com/owncloud/core/issues/25969
I vote for extracting the Dropbox code to an app of its own. Community developers will find it easier to contribute.
I remember that the original plan was to port most external storages to flysystem.
Currently the flysystem plugin for Dropbox is still on the v1. With a bit of luck it might be adjusted for version 2, so porting our Dropbox backend to use Flysystem could benefit from that.