Note that Rossen later followed-up in that same meeting to undo that resolution:
https://www.w3.org/2020/01/23-css-irc#T12-29-54
"RossenF2F: Re. merging, scrollbar-width/color just styles controls... scrollbar-gutter is more about the box model
... so let's not move everything to scrollbars yet until we know what will remain there"
So for now, this no longer impacts CSS Scrollbars.