scrollbar-gutter: force was proposed to reserve the same amount of space in a non scrolling element as the gutter would in an adjacent scrolling element, for the purpose of lining things up visually. See example 15 in the spec. However, this value has been controversial, for a variety of reasons, some listed in the spec, such as issue 45 or issue 46.

I think subgrid solves the same problem, in a much more robust manner, and we can therefore drop the proposal for scrollbar-gutter: force.
Here's a little bit of sample code demonstrating it:
http://software.hixie.ch/utilities/js/live-dom-viewer/?saved=12282

Overlay scrollbars	Classic scrollbars	Classic scrollbars with overflow:auto and no overflow

Everything aligns just as you'd want, regardless of overlay vs classic, overflow or not, scrollbar gutter or not…

So, since the use case that scrollbar-gutter: force was meant for looks solved, I propose we drop it.