I have been out of the loop for awhile, and didn’t pay attention to the move to Yarn. Are there any good tutorials, or walkthroughs for maintaining past Sage projects and migrating away from Gulp to Yarn? I have a whole workflow of about 8 sites, and need to keep those as is, but when I create a new project everything breaks.
Sage is a starter theme, not a theme framework, so there’s no standard upgrade path for “old” versions. Your theme is your theme; Sage was just where you started building it.
Updating a theme’s starting point from Sage 8 (which used Gulp) to Sage 9 (which uses Yarn) would be prohibitively difficult in a lot of cases. Templates would need to be moved and renamed, and updated to use Blade syntax.