Isn't Remix owned by Shopify? I'm fairly certain Remix is used on some of the Shopify platform / admin pages, that's a fairly large and well-used real world website being maintained in Remix
Shopify has had issues adopting Remix because their existing front-end codebases are massive and they couldn't afford to rewrite them to use Remix.
That's actually the whole motivation behind turning Remix into a bundler plugin. With a bundler plugin, you can incrementally migrate parts of your existing application to use the features of Remix without having to rewrite all of it.