Earlier quoted context omitted.
Something like Gerrit. Instead of carefully crafting a logical series of patches that are all well documented with commit messages, PRs are just garbage filled diff soup of "fix typo" commits. I hate it. It's hard to review and seems to be based on putting the least amount of effort into proposing changes to the code. See https://gist.github.com/thoughtpolice/9c45287550a56b2047c631...
That's down to culture and (self) discipline, not tools.
Something like linked and dependent PRs in a chain would go someway to replicating Gerrit but again this basic functionality is not available out of the box for whatever reason.