I think this proposal does an extremely poor job conveying the problem rhythmic sizing solves and why the solution is better than the "before" screenshot. I really don't understand it. I don't understand how I would configure the spacing beyond "CSS will keep the vertical rhythm across multiple columns" which seems extremely opaque and niche and I'd rather have implementers focus on one of the dozens of other things that CSS is objectively lacking and that people have asked for. But maybe I'm culturally biased and the 2nd screenshot is actually incredibly beautiful compared to the first one to someone more in tune than I.
And if I have to configure the spacing manually anyway, why wouldn't I just put `em`-based margins on my blockquotes or whatever.
The first draft is nearly ten years old at this point https://www.w3.org/TR/2017/WD-css-rhythm-1-20170302/
Does not look like too much has changed really. Still lots of unresolved issues. As noted elsewhere it is still very vague in terms of providing use cases, too.