I always find these discussions interesting mainly because of the "nobody knows" factor.
What I want to point out more than any one particular thing is that there are a limited number of 'ranking factors.' Whether that number is 200, 50, 3, or 3000, it's limited. Whenever one tactic loses its effectiveness simple math says that other factors increased in importance.
Google is very, very good at messing with SEOs.
If keyword density is limited or removed, something took its place. Something that isn't "create quality content." Most serious competitors are creating high quality, relevant content. If you think the difference in ranking one bank over another is "quality content" on a target page for "home loan" you're lying to yourself. Backlinks, age of the page, age of the site, internal links, anchor text, and some level of "keyword density" (though I think it's much more sophisticated than that" definitely helps. H1s still matter, as do H2s and H3s, tbh.
I have competitive keywords ranking with NO on-page content AND no backlinks. The page literally has a title, H1 tag and the surrounding menus & sidebars. It ranks for gambling-related keyphrases in a supposedly hard to rank niche and has ranked for months (with zero on-page text.)
SEMRush shows that same site rank for over 1200 keyphrases. It has ONE backlink to the homepage. That's it.
I wish that was the only example. But I am ranking semi-competitive marketing & SEO related keyphrases on a site with about 8 links and virtually no content. If content + links = SEO, these would never rank. So again, it's beyond that. Age of the domain? No, one is brand new. One is older. One is registered for more than a year, one for less. One is an EMD, one is not.
We've had new clients struggle & struggle to rank for really easy keyphrases with no backlink spam, technical on-site looks good and titles/content/links are all in line with other (ranking) clients. We put all their content on a new domain & it ranks just fine. NO links.
SEO is just weird. Let's face it - we're all attempting to do the best we can for clients but at the end of the day, none of it truly makes that much sense.