You just curl the site or use its API, if it has one? Then you store the result in a database and see if its value has flipped. I don't get the question; this is trivial.
How is that trivial in the general case?
How does it generalize arbitrary indications of status into yes/no?
How does it know how to use arbitrary APIs to obtain arbitrary indications of status?
one of the examples is to see if a new coffee shop is opened in town. what's the API to call for that?
You could use an LLM to pick the right API.