After Atto editor removal in MDL-83282, styles in atto.scss should also be removed. But because some content edited/generated with Atto added specific CSS classes, instead of removing atto.scss file, the styles should be deprecated instead. See SCSS deprecation process.

Apart from the SCSS styles related to Atto, there could also be some other Atto mentions that should be tackled (probably in follow-up issues).