This is a great synopsis that while directed at/inspired by WordPress spans far beyond that project into anything open source that’s built with CSS. Backwards compatibility is an interesting problem. So many people argue to not worry about it, that it does nothing but encourage people to not upgrade and it adds code bloat. I get that. I also get that systems are made from a ton of moving parts, and upgrading doesn’t always go to plan. I don’t say that to outline that I don’t upgrade my stuff, I do, passionately, but I can say that I’ve been burned in unexpected ways through the years and I imagine you have too.
It’s funny to think that when we talk about backwards compatibility it usually revolves solely around code that is processed on the server. Abstracting it further to the CSS layer really makes things interesting, especially with a project like WordPress.