Updates are in source repositories, and I think that's sufficient even though them available on modpack server would have been better. Anyway, we already avoid the extra work of making the same updates to the source repositories twice after branching.
Custom modpacks in default modpack.list should be relatively fresh at the time of S3_2 branching ( #44374 ), so that both ongoing branches would not need to do the same updates (up to compatibility with branching point)