Software and software-implemented inventions are not categorically disqualified from patent protection in the United States. As Roman Trusov has pointed out, you can find multiple examples of patents issued by the USPTO that are directed to ML algorithms. However, whether these types of patents would survive a post-grant challenge based on subject matter eligibility is uncertain. During litigation, an accused infringer would most likely argue that a patent claim covering an ML algorithm is invalid as directed to an abstract idea, and claims to mathematical algorithms are often vulnerable to this line of argument. The success of such an argument would largely depend on the wording of the patent claims and the manner in which the invention is framed.