204 204 No Content

The request succeeded but there is no content to return.

When you'll see it

When a request succeeded and there's nothing to send back, typically after a DELETE, a saved setting, or a CORS preflight request.

That the response has no body. Clients won't read one, and some treat it as an error.
That your JavaScript doesn't call JSON parsing on the empty response.
That no public page returns it. An empty page has nothing to index.

HTTP/1.1 204 No Content
Cache-Control: no-store

Not used for web pages. If a page returns 204, it will not be indexed.