You can manually stack PRs by making the merge target another branch.
The workflow just makes it a pain, since you have to manually rebase in both the branch and the UI after the original PR merges.
That doesn't work if the base and PR branch are in different repos, which is the most common way of doing things in Github.
That doesn't work if the base and PR branch are in different repos, which is the most common way of doing things in Github.