Isn't it normal to copy code with citations into your project? It also helps document what's going on.

Sounds like a codebase filled with the ghosts of programmer past. Just put the code in and own maintaining it. Anyone can do a git blame and find the history of its implementation, and also see the original PR which should have described what it was doing, and provided far more context to the reviewers than a code comment.

And if an architecture decision is so significant, retain a repository of architecture decision records.

I think you misunderstood what I mean. I'm talking about:

    // Credit Foo Project per MIT license
    // See <https://github.com./foo/project/path/to/file

    fn some_foo_fn() {

This has nothing to do with git history.