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.

That's all one change, because they redesigned it once! You can't just pick apart all the elements of the design to claim its multiple changes.

I agree the proportional scrollbar was good.

Many changes are still many changes even if they happened at the same time. You can't just lump together all the changes to the design to claim its only one.

I don’t have citations handy but I don’t think they changed it all at once. There might have a been a single major switch on macOS but I think all this stuff was developed incrementally on iOS.

What? Of course you can. Multiple things were changed. That they were all lumped together in one update is meaningless.

1 change to scroll bars. 1 change to something else. And so on.

You cannot restore the scroll bar width or buttons.