@proton.me some ideas for the Proton Drive Linux client

Windows has "Cloud Files API". Apple (macOS and iOS) has "File Provider". Android has "DocumentsProvider". Linux has nothing.
I use "On-Demand Files" all the time (Personal NAS), so this is the main blocker from me using Linux as a daily driver

You need the File Explorer to be aware of the Cloud Storage Provider to avoid unnecessary file reads (e.g indexing or file previews)
GNOME has their own File Explorer (Nautilus) as well as a Google Drive Integration (GNOME Online Accounts), so that would probably be the best thing for a PoC

There's an xdg-specs issue which talks about a potential implementations (FUSE Passthrough + xattrs), but I think the discussion died
gitlab.freedesktop.org/xdg/xdg-spec...

Valid Question. It would probably be a freedesktop standard implemented via FUSE.
This spec can then be implemented by Cloud Storage Providers (NextCloud, Google Drive, etc) as well as File Explorers (Nautilus, Dolphin, etc).