The idea that a license added would only apply to code added after the license change is very funny.
I suppose it makes sense if it originally had a license, and you then change the license to be less permissive.
What is more difficult is that earlier commits won’t have that license explicitly unless you rewrite git history to make that happen (which is possible but tedious).
I will probably not do that, but I guess it factors into my second question: That I in that case should make sure to include it in all branches (which are not treated as branches in the common sense, but rather as forks within the repo - they will never be merged to the main branch).