The CSS Scrollbars first public working draft (https://www.w3.org/TR/2018/WD-css-scrollbars-1-20180925/) was published, with as requested removal of “scrollbar‑3dlight‑color, scrollbar‑arrow‑color, scrollbar‑base‑color, scrollbar‑darkshadow‑color, scrollbar‑face‑color, scrollbar‑highlight‑color, scrollbar‑track‑color, scrollbar‑shadow‑color”.
Rather than a new 'overflow-style' property, see the CSS Overflow Level 4 (https://drafts.csswg.org/css-overflow-4/) specification for how it handles the related use-cases. I believe all consensus use-cases requested in this issue have been addressed, and this issue can be closed without any further edits.