I continue to think about CSS back-compat, especially as we dive deep into the land of MP6. Given that we are talking about build processes as well, I had an idea. There are two steps to this idea: a section (or separate file, if/when we get into building concatenated/compressed CSS for core) for CSS that is being kept for backwards compatibility. Things would generally stay for a cycle or so, but subject to evaluation in some cases for history and wide usage, such as settings tables. After that cycle, rules would move to a deprecated.css that is registered but not enqueued – if a theme or plugin needs it, they should enqueue it on applicable screens.
This is all just an idea – I don’t know if there would be hard lines about how long CSS stays in back-compat or deprecated areas, or if this would even work at all. I have no idea if we could or should throw notices somewhere (console?) like PHP can, and now jQuery with jQuery Migrate. Thoughts?