sass-loader 9.0.x & vue: ValidationError: Invalid options object. Sass Loader has been initialized using an options object

I recently upgraded my sass-loader from 8.0 to 9.0 and when I run npm run build (vue-cli-service build) I get this error

ValidationError: Invalid options object. Sass Loader has been initialized using an options 
object that does not match the API schema.- options has an unknown property 'prependData'

The only occurence of the word prependData is in my vue.config.js:

{
  //...
  publicPath: process.env.VUE_APP_ASSETS_PUBLIC_PATH,
  css: {
    loaderOptions: {
      scss: { // \/\/\/\/ here's prependData below
        prependData: `
            @import "~@salesforce-ux/design-system/design-tokens/dist/theme-one-salesforce.default.scss";
          `
      }
    }
}