yarn add -D react-app-rewired customize-cra

// package.json
"scripts": {
  "start": "react-app-rewired start",
  "build": "react-app-rewired build",
  "test": "react-app-rewired test"
},

// .babelrc
{
  "plugins": [
    ["@babel/plugin-proposal-decorators", { "version": "legacy" }],
    ["@babel/plugin-proposal-class-properties", { "loose": true }]
  ]
}

// config-overrides.js
const { useBabelRc, override } = require('customize-cra')

module.exports = override(useBabelRc())

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• Simplest Way to Install Babel Plugins in Create React App - DEV Community 👩‍💻👨‍💻

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CRACO로 구성하는게 지금은 맞는 것 같다.