As a Linux user I need a secure way to backup files to Proton Drive. I would prefer to use a tried and trusted tool with encryption support such as rclone or borg.

I regularly use rclone on my Google Drive it is an incredible app which would boost the capabilities of Proton Drive a lot.

This is critical, I want to use upgrade my account and use proton for mail / drive / vpn but cannot at the moment because there is no way for me to sync my files (primary OS is Linux).
Giving limited support to the RClone open source developers seems a no brainer; it is popular, cross-platform and robust. At the moment it looks like they are trying to sort bugs (example below) caused by essentially having to reverse engineer the proton drive API:
https://github.com/henrybear327/Proton-API-Bridge/issues/3

In few words, an secure API will be great to upload files over CLI on Linux / Unix using RCLONE

I am a Proton Unlimited ProtonDrive user.
I currently use rclone for Linux on Google Drive and it automagically works syncing my files from desktop to cloud. Please add rclone support in the next release. Very important for offsite storage.

this feature would be useful. I used to do backup of my documents with rclone. Thank you.

As a Visionary user (this email address is attached to my Proton account), I want this feature as well.

Although I have access to remote desktops with high network speeds, I want to automate the collection of some data for a project of mine so that 3TB of storage actually gets put to work.

Agree. To execute things "manually", using a GUI app or the web interface is not always a solution. Should be supported also the CLI way and scripting, wich can be more effective, simple and fast.

Documenting the API for drive is critical. Nobody wants to waste time reverse engineering how it works so that we can actually use our storage

In order for this to work, a public API is needed.

To either enable integration to Rclone or perhaps to develop a linux dedicated package to get better control (security is priority)