What happens when the URLs referenced by the README stop existing (domain expiry)? What about when the project is forked? What happens when I want to look at old documentation locally?

Why should the entire project not be subject to version control? It's a well established industry best practice.

Reproducible documentation is just as important as reproducible builds.

There's likely a strong argument to be made for Git sub-modules though i.e. putting the documentation in a sub-module.

This particular content isn’t documentation so much as marketing material—part of the shift GitHub has foist upon us in what a README is. But even were it documentation, that needn’t belong in the same place. This is the wisdom of rjzzleep’s suggestion of the wiki repository, an auxiliary Git repository that GitHub already creates for you.