See the following issues: https://github.com/webpack-contrib/sass-loader/issues/954 and https://github.com/sass/sass/issues/3065.

The quietDeps option isn't exposed yet to the Node.js API.

In the meantime you can downgrade to sass 1.32 without too many changes.

EDIT: It's now available in sass 1.35.1.

To hide the warnings, update your vite.config.js like this:

export default defineConfig({
  css: {
    preprocessorOptions: {
      scss: {
        quietDeps: true
      }
    }
  }
})

UPDATE: a critical requirement is that you have your paths relative to node_modules or another folder that SCSS is considering to hold external modules. These folders can be listed in loadPaths scss option (node_modules is included by default, at least when SCSS is run from Vite). And not relative to current file!

E.g. this path works - all warnings are excluded:

@forward 'spinkit/scss/spinners/7-three-bounce.scss';

This path doesn't work - all warnings are shown:

@forward '../../node_modules/spinkit/scss/spinners/7-three-bounce.scss';

This requirement can be understood by reading SCSS quietDeps option description.

It works fine for me, like this:

css: {
    preprocessorOptions: {
        scss: {
            quietDeps: true
        },
    }
},

This seems to be an 'internal nextjs thing' that you should not be too concerned about. Once they migrate it 'under the hood' there may be some impacts in a nextjs release.

You mentionned not wishing to silence it, but in case you change your mind, there is this option in your next.config.js :

 sassOptions: {
    silenceDeprecations: ["legacy-js-api"],
  },

Read more on this here : https://github.com/vercel/next.js/discussions/67931 and here : https://github.com/vercel/next.js/issues/71638