A word people who want to sound intellectual™ use for “supporter” when they are embarrassed by “apologist” or “sympathiser”.
The concept of an ally is as old as humanity, but typically refers to situations involving conflict (predominantly in relation to nation states). However, the pulpit-rivalling, self-aggrandizing and ego-stroking modern version has its roots in the concept of “privilege” and the theories of “oppression”. Although a flood of dreadful papers start to appear around 2000, the most likely source is “Beyond Tolerance: Gays, Lesbians, and Bisexuals on Campus” (Nancy J. Evans, Vernon A. Wall, 1991).