TL;DR: The gnome-shell-sass repository is no longer getting updated upstream.
As gnome-shell’s CSS grew more complex, designers needed something more expressive, so they started compiling the stylesheet from SASS. The sources were moved to a subproject, so they could be shared between the regular stylesheet and the GNOME Classic stylesheet.
Fast-forward to the present day, gnome-shell now includes a light variant to support the global color-scheme option.
GNOME Classic has been updated to use that built-in support instead of compiling a separate stylesheet on its own.
That means that the gnome-shell-sass repository is no longer needed by the gnome-shell-extensions project, and will therefore no longer be updated upstream.
If you are building your own theme from those sources, you will either have to get them from the gnome-shell repository yourself, or coordinate with other 3rd party theme authors to keep the subproject updated.