Thule

This extension sends the content of your active tab (either automatically or when you press a button) to large language model API so it can check if this is something that might waste your time.

You define a prompt like "Please block all pages that just seem like mindless surfing. I am trying to study computer science." and it will do its best to block these pages.
The blocking is really just a popup asking you if you want to close the site combined with reasoning as to why it does not fit with your prompt.