I can understand your concern and yes I have faced a similar situation when GitHub recently took down "YouTubeDL Project" thank god that I had my mirror running. and was able to share the source with the community. 
Moving out of Github is not a good idea for Small / Upcoming Community Based Projects.
But I am not aware of the XCP-NG Community yet as I just started using it.
And my recommendation to it will be to run your own MIRROR using Gitea which is lightweight and easy to use based on my experience
Again MOVING OUT OF GITHUB IS NOT A BETTER CHOICE
if in case you decided to run a mirror then go with Gitea and if needed you can use a small mirror script I created
Python Mirror Script: https://github.com/varunsridharan/github-gitea-mirror
Github Action: https://github.com/varunsridharan/action-gitea-mirror