(The license is ~14 years older than git itself. There’s no way distribution using a specific git hosting service is required under the license.)
GitHub is free to change their minds about the implementation and start accepting those commits again. That is, you, as a recipient of the code, are free to ask GitHub to host it, and to offer them a license under the terms of the GPL; GitHub is free to accept or to decline, same as if you sent me the code, I am free to host it on my personal website but I'm also free to not host it.
(It would feel more like violating the spirit if it, say, prevented you from uploading it to a GitLab instance you self-host... but since you self-host it, you could just patch out any suppression code, so that case can't come up.)
There are all sorts of compatibility issues that can arise from platforms doing dumb things. For example, a repo with the files README.md and readme.md in the same directory would break on certain Windows machines due to case sensitivity problems.
It also feels like it would violate the license of the repository. I see that one of the author's repos is LGPL [1], which allows redistribution of the source. Restricting how that source is distributed seems to me like a violation of that license, if not in letter at least in spirit.
(Of course, it goes without saying that I'm not a lawyer, which is why I said it feels like a violation.)
1: https://git.joeyh.name/index.cgi/haskell-mountpoints.git/tre...