Responsive design has been standard practice for over a decade. Some of the original guidance is still valid. Some has been superseded by better tools. And some issues that didn't exist in 2015 are now significant.
Still Matters
The mobile-first default is more relevant than ever — global mobile web traffic has only increased. Performance budgets matter more than ever on mobile networks. Accessible tap target sizes (minimum 44x44 points) are just as important as they've always been.
Superseded by Better Tools
Fixed breakpoints at specific pixel values (768px, 1024px, 1280px) have been largely replaced by component-level responsive design using CSS Container Queries, which let components respond to their own container size rather than the viewport. This produces more reusable components because they work correctly wherever they're placed.
Responsive images have a better solution: the srcset attribute and the <picture> element, combined with modern image formats (WebP, AVIF), do more than anything from 2015.
New Issues
Foldable and large-screen phones have introduced display sizes that old breakpoints don't account for. Variable viewport height on mobile browsers (where the address bar shows/hides) breaks layouts that use 100vh — 100dvh (dynamic viewport height) is the current fix. The proliferation of notches, punch-holes, and rounded corners requires safe-area-inset CSS environment variables.