For using Dropbox as a special remote, there are currently several choices:

• rclone supports the Dropbox API, so the rclone special remote can be used.
  (Actively maintained)
• Alternatively, there are some dedicated special remotes:
  • https://github.com/TobiasTheViking/dropboxannex
    (Last updated 2014)
  • https://pypi.org/project/git-annex-remote-dbx/

At this time it's not clear which is better, so if you find one works best, please comment below.