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...
Well but this is controllable, i.e. it is people who choose to do this not the platform. Very much an internal design choice.
That's down to culture and (self) discipline, not tools.