Hi. I'm a starter theme called _s, or underscores, if you like. I'm a theme meant for hacking so don't use me as a Parent Theme. Instead try turning me into the next, most awesome, WordPress theme out there. That's what I'm here for.
And since that there is a layout folder in the SASS one, and that _s has now build tools, theme developers can include one of the two layouts in the final style.css by uncommenting one of these two lines.
Back in the day when the layout folder has been introduced https://github.com/Automattic/_s/commit/5cad5adab9cfb4ec352d6d102504a0cd90e396d3, it served as a copy/paste resource for theme developers to start building their themes. Since then, this folder remained mysterious. Some didn't know how to use it (https://github.com/Automattic/_s/issues/407), others thought it was a bug (https://github.com/Automattic/_s/issues/1373).
@grappler suggested in 2016 (https://github.com/Automattic/_s/issues/945) that he saw a number of themes in w.org that don't use this folder and doesn't remove it, and @carolinan confirmed that this is still the case (https://github.com/Automattic/_s/issues/1387) and that 99% of themes don't use it.
And since that there is a layout folder in the SASS one, and that
_shas now build tools, theme developers can include one of the two layouts in the finalstyle.cssby uncommenting one of these two lines.