Since the beginning it has not been possible to specify an expected HTTP status code (though it was possible to specify a string to search in the body response, which often allowed to circumvent this limitation).
Well it is now possible to specify an expected HTTP status code (other than the default 2xx), in the same place you could specify a string to search, the tooltip has been changed accordingly:
This can be useful if you need to monitor an endpoint with a non 2xx status but which doesn't return any body (or not stable enough to match on). The error message will be the same for now: UP/DOWN (short version) or "Couldn't find the configured string" (long version) because it's simpler. I'll see about changing that a bit if it creates confusion.