Use it but don’t rely on it. Celeste uses rclone. The rclone support was temporarily disabled from Proton’s end a while back and also, the rclone backend still has a bunch of bugs and the developer seems to have gone missing

They’re the maintainers of the most popular JS PGP library and they’re pushing for some major upgrades to the PGP standard. There’s no competing standard. Proton is pretty much the only popular encrypted mail provider that actually does interop well enough.

Or maybe they just want to disclose as little of their personal information, including services relied on, on an open platform like this. Idk if that’s the case, but playing devil’s advocate here