I'm on a long weekend. This did not prevent git-annex from getting an impressive lot of features though, as Daniel Dent contributed https://github.com/DanielDent/git-annex-remote-rclone which uses rclone to add support for a ton of additional cloud storage things, including:

Google Drive, Openstack Swift, Rackspace cloud files, Memset Memstore, Dropbox, Google Cloud Storage, Amazon Cloud Drive, Microsoft One Drive, Hubic, Backblaze B2, Yandex Disk

Wow! I hope that rclone will end up packaged in more distributions (eg Debian) so this will be easier to set up.