Why

1. Sage doesn't come with Purgecss (or uncss, etc.)
2. Say, we use Bulma -- suddenly dist/main.css grows by 400K!
3. Let's purge it!
4. Oh dang, we need to whitelist /\.wp-/, /\.post-type/, /myfancylightbox/...
5. Wait we are whitelisting pretty much everything from /resource/assets/styles!
6. Isn't there an option to purge /node_modules/**/* only?
7. Nope.
8. purgecss start ignore, strategically placed, to the rescue!

Now, this might not be 100% effective, but it is very convenient. YMMV!

Installation

yarn add -D purgecss-webpack-plugin
yarn add -D glob-all

Set up Webpack plugin

// resources/assets/build/webpack.config.optimize.js
const glob = require('glob-all');
const PurgecssPlugin = require('purgecss-webpack-plugin');
//...
module.exports = {
  plugins: [
    //...
    new PurgecssPlugin({
      paths: glob.sync([
        'app/**/*.php',
        'resources/views/**/*.php',
        'resources/assets/scripts/**/*.js',
      ]),
    }),

Configure Sage's PostCSS

// resources/assets/build/postcss.config.js
// we need to keep "important" comments
const cssnanoConfig = {
  preset: ['default', { discardComments: { removeAll: false } }]
//                                                    ^^^^^

Edit main.scss

// resources/assets/styles/main.scss

@import "common/variables";

/** Import everything from autoload */
@import "./autoload/**/*";

/**
 * Import npm dependencies
 *
 * Prefix your imports with `~` to grab from node_modules/
 * @see https://github.com/webpack-contrib/sass-loader#imports
 */

/*! purgecss start ignore */
// note the exclamation mark, which makes this comment "important"!
// everything above this line will be purged!
// everything below it will land in dist/main.css!

/** Import theme styles */
@import "common/global";
//...