data-bs-backdrop

  <button type="button" class="btn btn-primary" data-bs-toggle="modal"
      data-bs-target="#staticBackdropLive">×でしか閉じないモーダル</button>
  
      <div class="modal fade" id="staticBackdropLive" data-bs-backdrop="static" data-bs-keyboard="false" tabindex="-1"
    aria-labelledby="staticBackdropLiveLabel" aria-hidden="true">
    <div class="modal-dialog">
      <div class="modal-content">
        <div class="modal-header">
          <h5 class="modal-title" id="staticBackdropLiveLabel">Modal title</h5>
          <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
        </div>
        <div class="modal-body">
          <p>I will not close if you click outside me. Don't even try to press escape key.</p>
        </div>
        <div class="modal-footer">
          <button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Close</button>
          <button type="button" class="btn btn-primary">Understood</button>
        </div>
      </div>
    </div>
  </div>