This sounds like an easy decision, Github is non-free, owned by what many consider an evil
company.
But, KaOS has benefited since 2013 (and some parts of the used code even longer) tremendously from all that Github has to offer, not in the least, complete stability.
This question has come up in IRC/Matrix. The discussion is not going very far, so maybe there will be more participants here in the forum.
A good article pointing to KaOS should move:
https://drewdevault.com/2022/03/29/free-software-free-infrastructure.html
Things to consider though. Self-hosting is not really an option, KaOS uses one server owned (local, real hardware), and has one server offered by Fosshost. To make sure that if one server goes down (even a short time), it does not effect all infrastructure, code should stay away from these 2 servers.
That really leaves only Codeberg as a viable alternative.
Gitlab is not really fully opensource either, and you can expect it will be sold to the highest bidder at one time, just like Github was.
Moving away from Github will not only bring a decent amount of work to get all the code migrated, but will also require code changes to the online package viewer, makepkg used in the build chroots and will no longer supply a full visible backup of the main website. Not many offer the site compilation of Jekyll sites like Github (KaOS website is build and maintained using jekyll).
So, opinions please, time to move, or better go for stability and stay with Github?