The official apps are all open source (GPL-3.0, https://github.com/ProtonDriveApps), so even though a formal API isn’t documented it is derivable from the 4 available clients. This is how rclone added support for Proton Drive. So there isn’t really anything stopping GNOME or KDE from supporting it.
Lobby the mods of those communities to have and enforce rules defining what must be tagged NSFW.