Like for people or are using a ad block or for a crawler downloading your content so it can be used by an AI response?

Arbitrarily, as in for any reason. It's your site, you decide what constraints an incoming request must meet for it to get a response containing the content of your site.

>and the website should not be notified about it.

My user agent and its handling of your content once it's on my computer are not your concern. You don't need to know if the data is parsed by a screen reader, an AI agent, or just piped to /dev/null. It's simply not your concern and never will be.

This article is about Cloudflare attempting to deny Perplexity access to their demo site by blocking Perplexity's declared user-agent and official IP range. Perplexity responded to this denial by impersonating Google Chrome on macOS and rotating through IPs not listed in their published IP range to access the site anyway. This means it's not just "you're free to deny access to your site arbitrarily", it's "you're free to play a cat-and-mouse game indefinitely where the other side is a giant company with hundreds of millions of dollars in VC funding".

The comment I'm responding to established a slightly different context by asking a specific question about getting compensation from site visitors.