All of Proton’s web clients were moved to this repo:
I’m not sure about their native clients, but IIRC they are in Beta and so their source may not (yet) be available. Don’t quote me on that though, I didn’t look too hard (but if that is a concern to you we can look into it further).
As was the case with past Proton releases, [the source code for mobile Proton Drive apps] will arrive a little bit afterwards when we have the bandwidth to address community contributions and answer issues which are opened. Most likely before end of this year  once we get through the initial rush of quickly pushing out the most requested improvements to the apps.
This is reasonable, this is how Proton has handled open-source releases in the past, so I don’t think they would stop releasing open-source code now.