I suspect that GNOME will focus on GNOME OS, and possibly support other downstreams whose GNOME integration is maintained by GNOME developers. This would leave third-party packages of GNOME software unsupported.

This is consistent with what I have seen from the GNOME project, but is NOT a criticism of the project. It's just the approach they have chosen to take.

KDE takes a very different approach and works much more closely with downstream distros, so long as those distros ship up-to-date KDE packages. In fact, my (potentially wrong) understanding is that KDE bugs in Fedora packages either should be sent upstream or are triaged by KDE developers. I forget which one.

I believe this is a philosophical difference. KDE is much more customizable, whereas GNOME focuses on having a single user experience that works everywhere.