Well this is it. What really enforces the policy is rejecting commits that break user space.
Now if you’ve got a large enough group of devs, rejecting commits is fine, but if you’ve only got a small group you need everyone to be working productively, and you can see why Linus ended up giving angry feedback about commits that were wasting everyone’s time.
It’s half this, and half an explicit policy “we do not break user space”. Together it meant that if you did anything that screwed up the user space you got told about it at length.
Now Linux culture is established enough that it only really needs the policy, and not the cussing people out to enforce it.
Famous email about it here: https://linuxreviews.org/WE_DO_NOT_BREAK_USERSPACE
It’s super hard to get involved as a UI person. If you’re a developer, you can just rock up to a project and fix bugs, and if you follow the coding style they’ll probably get accepted.
If you want to successfully contribute as a UI person you have to convince a bunch of developers that you know what they should be doing better than they do. It basically never happens.