It's kind of sad that this is true.
I'm guilty myself, I contribute to projects on GitHub more often than on any other platform.
And when I search for open source projects the first page I use is GitHub.
They had a sizable lead and completely bungled it.
While SF was crapping where they eat, GitHub built a lot of trust and goodwill with a lot of people.
I would consider SF a viable Github alternative, but the bad reputation caused by a temporary owner just seems to stick forever.
That ancient feeling UI doesn’t win them a lot of forgiveness, if the only notable change has been “we took away the malware” and the site continues to remain stagnant, SourceForge will continue to feel very inferior to more modern alternatives.
Some people don't even know the difference between Git and GitHub...
I have also found this to be the case, even with engineers that have years of experience. It's both impressive and awful.
That said I do feel some joy when I see a project on Gitlab and am happy to contribute there, eg FDroid.
Maybe Copilot (made possible by the huge non-commercial codebase on Github) being somewhat unfair advantage to other commercial alternatives is a bit troublesome, yeah. But otherwise I just don't see why Github being a de-facto standard is bad. In fact, I am somewhat annoyed when a really popular project doesn't have a Github repository (mostly because it makes filing an issue, or even reading existing issues much more difficult in most cases). So I'm actually glad to hear that some big projects feel pressed to migrate to github. What's even a problem with that, apart, maybe, for github actions, that honestly suck?
(Maybe I should add: I am a git hater, and do think that mercurial is just unquestionably better, but this battle is lost long time ago, so I don't suppose it's the topic of this discussion.)