This would be great for use with Cryptomator, thanks.

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

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

NextCloud, Enpass, Joplin... it seems every bit of software that allows me to move away from trusting free services with my data support Webdav as one of the key technologies for easily synchronizing my data between devices.

There are a number of services I use that support WebDAV for remote storage and synchronization. Support for WebDAV in ProtonMail would allow me to store my app data securely and privately and avoid those non-private services. For example, EnPass is a password manager that doesn't store your data online, but allows you to store it in cloud services, including those that support EnPass.

And to save other important backups like password managers databases.