Ok so, previously it was a case of just using "data:"But, since Vue 4, it seems it now has to be "prependData:"
module.exports = { css: { loaderOptions: { sass: { prependData: `@import "~@/styles/_variables.scss";` } } }}
it's changed to additionalData
additionalData
module.exports = { css: { loaderOptions: { sass: { additionalData: `@import "~@/styles/_variables.scss";` } } }}