Do you mean 100% external CSS, or just any external? Because I've definitely seen teams implement the design system as external CSS, then adjust at the component level within the code. I've done 100% external CSS on private app with a few hundred users, but nothing public. I currently have a side project that uses React for content management, and builds static HTML with external CSS via an SSG for publishing.

So... it happens. But I use SPAs for internal apps, not public web sites, so despite my knowing such things exist, I got no links to share.