Nested H1s was never semantically correct in the first place, at least for accessibility purposes.
You can do flexible sizes without media queries (eg, viewport size units + clamp). Designers generally understand the web pretty well these days.
I only see one situation where people might have depended on these styles, but it's a big one - anywhere that you output the plain HTML of a "rich text" component from a CMS or whatever. There, if the stakes are low, it might not have been a big deal to just let the browser do it and headings might look too big sometimes now.
I can't articulate it and I could very well be on the wrong vibe, but this feels like the bad practice of relying on the failure mode within a try/catch for normal functioning.
This is not about graceful degradation. An HTML document without any CSS should never be in a degraded state. It should be perfectly usable and perfectly well accepted.
However, if CSS specifies some things and omits others that are related to it (one example is specifying the font size for one heading level but not another one; another example is specifying the background colour without specifying the foreground colour or vice-versa), then it makes sense to be a warning.