With the following versions in package.json:

    "@typescript-eslint/eslint-plugin": "4.9.0",
    "@typescript-eslint/parser": "4.9.0",

I get a warning that is reminiscent of an issue fixed in typescript-eslint 4.1.0 (typescript-eslint/typescript-eslint#2496)

With this in a .d.ts file:

declare module 'test' {
  interface Test {
    test: 'test'
  }
}

I get the warning: 'Test' is defined but never used. (eslintunused-imports/no-unused-vars-ts)

There were adjustments to no-unused-vars in typescript-eslint 4.9.0 that may have broken something. (typescript-eslint/typescript-eslint#2768)

Locking my typescript-eslint version to 4.5.0 (consistent with this library) resolves the issue, but thought I would note what I've experienced.