Though the OSR is a little clearer, you can still run into issues where some people will at least defend it applying to retroclones of games of the same era (Cepheus Engine to pick a particularly stark example since unlike some of the others its not based on a one-time TSR product), but it gets particularly gritty when the people who expect "Old School" in general to only apply to D&D, with others using it, as you note, for things like RQ, Traveler, and Villains and Vigilantes.
(For particular fun, in a group about that, ask them if Champions (originally published in 1981) is Old School in the earlier versions (and make no doubt, there's a distinct sort of cultural break between people who never liked a version later than 4e (or sometime 3e) and people happy playing 5e or 6e); you can sometimes get resounding silence).