I opened my RSD 1.5 Visions Turtorial I completed in July of 2016 and it imported using the import methods they have defined for V3.
There are no more rows or columns (None) in the vanilla scheme. You design only with containers to use flex (rows and columns) or css grid (rows and columns and beyond the unthinkable). Totally slim and future proof.
This really should be attractive to designers who don't need the Bootstrap or Foundation Love.
Great addition.
Bootstrap 5 CSS Grid.