shmerl: lgogdownloader supports Galaxy API already, but it's now only exposed for Windows builds of games. Linux builds are excluded from it.
Since I don't think the GOGBear or his prophets will reply to your queries, I'm going to take a shot at some of the points you've raised :).
I've never actually checked lgogdownloader's source code, but if it's using the Galaxy API I think it's using (there aren't that many choices anyway), then Windows and Mac builds of games should be supported. At least that's what the said Galaxy API provides information on.
Now there are other Galaxy API endpoints which provide download details for Linux installers as well, so I don't see any major roadblocks in adding this functionality to lgogdownloader even as things stand now (with a bit of a headache, most likely treating Linux games as an entirely different kind of beast).
But yeah, the support for Linux games in general clearly does not span the entire API space. Which is why a Linux version of Galaxy is probably not even in the works - there isn't even wide backend support for it yet.
shmerl: GOG are decommissioning the legacy API.
I certainly hope that's not the case. As far as I'm aware the GOG Downloader used a specific (legacy) format for the download URLs, which is what, I think, is being cut from the backend.