For #2850

From https://docs.djangoproject.com/en/5.1/releases/3.1/#deprecated-features-3-1:

The HttpRequest.is_ajax() method is deprecated as it relied on a jQuery-specific way of signifying AJAX calls, while current usage tends to use the JavaScript Fetch API. Depending on your use case, you can either write your own AJAX detection method, or use the new HttpRequest.accepts() method if your code depends on the client Accept HTTP header.

If you are writing your own AJAX detection method, request.is_ajax() can be reproduced exactly as request.headers.get('x-requested-with') == 'XMLHttpRequest'.