Gregor, if I understand you correctly, you are implicitly saying "please put your stuff on one of the big sites like github/gitlab/bitbucket".
I personally think that this is the wrong direction to move, as it makes the Internet more dependent on a few entities. That makes it less robust, as we have seen in the censorship case at github related to nationality.
IMHO, this does not apply to Git repositories. It is very easy to leave public forks in several places, and there are even ways to automate pulling from one repository to the other.
Private Git hosting has its merits, but it makes it more difficult to send patches or improvements. There are ways around that of course (good example: the Linux kernel), but they rely on a lot more effort than is usually necessary for a small open-source project.
Of course, it is everybody's own choice to not use public collaboration platforms, but there is also not much harm in doing so. Should one of them shut down, crash & burn, or change their terms of service, there are always other ways to share repositories.
This is a very different matter than social networks that rely on proprietary protocols and infrastructure.
I understand your point that it should be easy to contribute, but maybe it is a more sustainable way to fire up your own git service and have your code pulled in from your machine, preferable via IPv6?
That sound like a tremendous amount of effort in coordination and setup compared to the benefit and will probably put off 99% of all potential contributors... Well, notwithstanding everybody on this mailing list, of course. ;)