The default mini-css-extract-plugin + css-loader, when run with webpack --production, seem to tree-shake css module classname mappings and inline the referenced names directly as strings.

index.js

import styles from "./styles.css";
const baz = styles.baz;

console.log("classname for baz", baz);

main-sans-webpack-boilerplate.js

// output of webpack's main.js with boilerplate removed. 
// No css mappings appear here, and only the value of our referenced classname ('baz')
// shows up in the bundle at all
function(e, t, r) {
  r.r(t);
  console.log("classname for baz", "_1Kxdli63PQyf_5b_GTMl04");
}

main.js

// output from webpack, note that unused classnames do not appear
!function(e){var t={};function r(n){if(t[n])return t[n].exports;var o=t[n]={i:n,l:!1,exports:{}};return e[n].call(o.exports,o,o.exports,r),o.l=!0,o.exports}r.m=e,r.c=t,r.d=function(e,t,n){r.o(e,t)||Object.defineProperty(e,t,{enumerable:!0,get:n})},r.r=function(e){"undefined"!=typeof Symbol&&Symbol.toStringTag&&Object.defineProperty(e,Symbol.toStringTag,{value:"Module"}),Object.defineProperty(e,"__esModule",{value:!0})},r.t=function(e,t){if(1&t&&(e=r(e)),8&t)return e;if(4&t&&"object"==typeof e&&e&&e.__esModule)return e;var n=Object.create(null);if(r.r(n),Object.defineProperty(n,"default",{enumerable:!0,value:e}),2&t&&"string"!=typeof e)for(var o in e)r.d(n,o,function(t){return e[t]}.bind(null,o));return n},r.n=function(e){var t=e&&e.__esModule?function(){return e.default}:function(){return e};return r.d(t,"a",t),t},r.o=function(e,t){return Object.prototype.hasOwnProperty.call(e,t)},r.p="",r(r.s=0)}([function(e,t,r){"use strict";r.r(t);console.log("classname for baz","_1Kxdli63PQyf_5b_GTMl04")}]);

styles.css

.foo-bar {
    color: red;
}

.baz {
    color: green;
}

.delete {
    background-color: hotpink;
}

.unused {
    color: purple;
}

.another-unused {
    background: white;
}

webpack.config.js

const MiniCSSExtractPlugin = require("mini-css-extract-plugin");

module.exports = {
  devtool: false,
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          {
            loader: MiniCSSExtractPlugin.loader,
            options: {
              esModule: true
            }
          },
          {
            loader: "css-loader",
            options: {
              modules: true,
              esModule: true
            }
          }
        ]
      }
    ]
  },
  plugins: [new MiniCSSExtractPlugin({})]
};