The quilt tool by Andrew Morton is one of these tools used for maintainance (https://man7.org/linux/man-pages/man1/quilt.1.html) of the kernel. It's actually quite nice to use. Early versions of git and mercurial had tooling around similar workflows on top of git/hg repositoires (guilt, being one of these tools).

My understanding is that even during the BK years, some maintainers refused to use it and would continue to use tarballs + patches.