@jdstrand, Note that these people are not using the kde-frameworks-5-core18 snap directly, but this simply gets pulled in because they use the kde-neon extension. This adds to their confusion and frustration.
Although I agree with @damir that this situation is very confusing, I have tried to warn publishers from using the kde-neon extension by putting the following notice on top of the kde-neon extension documentation:
This extension is currently only available for the KDE project itself. Snaps from non-KDE publishers will not get automatically connected to the KDE runtime snap required by this extension.
I understand and respect the decision to only auto-connect content snaps when the author of the content snap agrees to that. Otherwise, we would end up with a bunch of snaps suddenly breaking because a content snap breaks binary compatibility.
However, I think this issue could have been handled better by the Snapcraft team. It is very confusing for publishers that an official Snapcraft extension can only be used by a single publisher, especially since the gnome extension is intended to be used by everyone. I asked about the plans for the Gnome and Neon extensions back in Oktober and nobody raised the issue that the Neon extension was only going to work for the KDE Neon project. So I wrote documentation on how to use the Neon extension, which was even mentioned in an Ubuntu blogpost, but even the Ubuntu blogpost did not have a clear warning that this extension would not work for snaps published by other people.
So I was very surprised that when I contacted the Snapcraft developers about the content snap not being auto-connected, they told me that this was not a mistake, that this was deliberate…