This backend uses the Proton-API-Bridge, which is based on go-proton-api, a fork of the official repo.
According to that page there was an official API library, provided by Proton. They forked it and added features, it didn’t need to be reverse engineered.
https://github.com/rclone/rclone/blob/master/docs/content/protondrive.md#-icon-fa-fa-folder--proton-drive
There’s no api, or there was, when rclone implement it.
According to that page there was an official API library, provided by Proton. They forked it and added features, it didn’t need to be reverse engineered.