It is common in many flows to expect people to do a force push if something is wrong with their pull request, e.g. having 15 commits that look like:

    Try stuff
    Another prototype
    Fix something
    ...
    Revert prototype

It is fairly common (in my experience) for the contributor to be asked to squash those commits.

In my opinion, the more guards in place the better.

I think that's a key point, the earlier you can catch the problem the less frustrating for both sides. There is https://www.npmjs.org/package/commitplease for commit hooks but it's still handy to have something audit pull requests.