As noted in other comments, any open standard is good. Anything you can already mount on Linux, presumably via a bridge.

proton-drive is pretty much useless without a programmatic way of moving data to it from Linux and Truenas. Rsync or ssh or webdav please. As others have said, moving files by hand through a web browser is a shitty way to do backups. Especially with all the warning dialogs about 'files too large' and 'don't stream at the same time'.
This can be done in a roundabout way with the windows proton drive app running in a vm via a shared folder with the linux host but I don't trust microsoft enough for that kludgy method.

Will upgrade from free to ultimate when webdav is implemented. Now using strato hidrive with webdav

Any comment, when you will implement WebDAV?

Absoltue must. Same reasons as people stated here before me.

I'd like to eventually migrate from OneDrive, but it won't happen until I can map ProtonDrive to my local file system (Win).

Need WEBDAV for Sync and as TeamDrive-Server (Storage). I have a paid plan for proton products.

I need to automate backups to proton drive to make it useful. From truenas or synology nas. Webdav would work. So would rsync. Having to upload files one at a time using a browser interface is a horrible way to do things.
Both Synology nas and Truenas support multiple cloud storage vendors but proton drive is not among them because of this missing feature. I am keeping a paid proton plan and praying for this problem to be fixed.

As long as Webdav is not possible I will continue to use free versions of Proton. If Webdav is possible I will become a paid user for Mail, VPN and Drive. This way I stop with services at other providers.

I support the idea for WebDav Support on Proton Drive. Please implement the WebDav Support.

This would be great for use with Cryptomator, thanks.

I concur. I have been looking into switching from Google Drive to Proton but I need it to sync with my Synology NAS which is still not possible.

So I have to stick with the free Proton Plan and Google for now. That's kind of a shame.

Also like to note like some people have said, WebDAV is not dead and most self-hosting services or apps use or have WebDAV syncing

I will like WebDAV support for proton drive as well. Right now I use Nextcloud for such things, but I want to use webdav for somethings. Also have to ability to select only certain folders to have webdav access too, not the root folder like most webdav apps.

E.g. to save important backups like password managers databases.
Note: there are more, similar requests for the same: WebDav support.
Votes can be added up

Really important. No reason to subscribe to protondrive plan without it.
Have the free plan now to test, works great.

ProtonDrive is useless without WebDAV or a CLI tool. Because I signed up early to the Visionary plan, Proton has given me multiple terabytes of storage for free. But it sits unused because I can't script backups to it.

This would also allow Proton Drive to be used for attachments by Thunderbird.

If you plan to store passwords from password managers using your own storage service this is a requirement

Definitely useful for me, want to run my backups, calendars, etc through it