I really love the project structure of C++. I know that it is an archaic design developed like this due to lack of resources, but I find packages extremely offputting.
The first reason is that splitting declaration and implementation across files makes it easier to figure out what something does.
Second reason is that I feel that I have more control over libraries and packages that have to be manually added to a project rather than using a package manager.
Third, I feel like modern languages iterate over too many versions too fast. C++ has version releases too, but I feel that versioning is handled better from time, compatibility and stability point of view.
GitHub nukes forks when the original repository is deleted. The correct way to handle your use case is by creating pull mirrors, ideally on a different host.