On macOS at least, it's not one change, it's at least four:
- scrollbars overlaying the content, rather than having dedicated space
- scrollbars that auto-hide when not in use
- scrollbars that are initially narrow, then widen when you mouse-over
- removal of the scroll buttons
That's not including the visual design changes, which are arguably important too (e.g. the scroll "thumb" now has a very flat design rather than textured so it looks draggable).
Edit to add: oh yeah, one more I forgot: in classic Mac OS the scroll thumb had a fixed size, now it reflects the size of the viewport. That one is a useful and positive change, though. Not sure where that innovation came from but it wasn't the Mac.