Because URM is defined by the AAMC as pertaining to racial and ethnic groups, most schools don't bother posting their own definition unless (like Northwestern) they care enough about recruiting LGBT doctors to expand it.
However, sometimes schools will post something elsewhere in their admissions process about LGBT or LGBTQ+ students or diversity, so just look at where you're applying. Some people who choose to mention sexuality, orientation, identity, etc. in their applications approach it from the lens of being a member of an underserved population (which LGBTQ+ definitely is) as opposed to underrepresented.
Medical College of Wisconsin and University of Minnesota, had whole sections of "do you ID as LGBTQ+? Explain." Not sure if that means you are considered URM, but they definitely examine it as a separate category.