Here:

if the options file is an ESM module, i.e.

export default {
  plugins: [...],
  presets: [...],
};

that require results in the object { default: { plugins: [...], presets: [...] } }; while previously (when the options file was CJS, exporting options like module.exports = { ... }) it would be just the options object { plugins: [...], presets: [...] }, so that breaks everything that depends on that helper. I believe, the same problem is present in other places where require, created via createRequire() is used.

P.S.: So, the actual way I bumped into it was attempting to migrate tests with @babel/helper-plugin-test-runner to full ESM version.