When discussing the OSR movement, does it only apply to games that emulate early D&D? Or do other early games such as Traveller or Metamorphosis Alpha also qualify as "Old School" especially as it relates to talking about OSR games?
This is a popular argument these days - I think it's the result of a few things, none of them good.
First, the OSR as a living scene or movement is done. There is no central place where OSR ideas are discussed and instead a large number of splinter communities have formed. It's like other art movements in this way - you can still enjoy OSR things, but the movement itself is done, everything now inspired by it is a product of one or another "Post-OSR" scenes. There are several of these, and many of them don't like each other. Many also claim to be the real OSR. None are.
Second, the most reactionary (often politically as well as intellectually) of these Post OSR movements, based on 4chan and a few blogs that use a lot of slurs and complain about how everything else isn't OSR, tend to claim that only D&D or even only AD&D are proper OSR games. This is historical revisionism, because at the time it was going strong the OSR was open to both playing and iterating from a lot of older games especially Traveller, Boot Hill, Palladium (RIFTS/TMNT) and Gamma World come to mind.
Third, people, usually wanting to try something new after 5E, look into the OSR and often stumble about because the wreckage of the OSR is hard to navigate. They both see that all the larger name "OSR" (many are actually Post OSR) games are D&D clones and that some loudly proclaiming themselves OSR say the movement
is entirely about playing games old D&D (or games very closely modelled on it). This issue is compounded by the assumption that many from the contemporary trad scene (5E/PF2 etc) bring to RPGs that playing by the book is essential. Couple RAW enthusiasm with revisionists defining the OSR purely as a nostalgia and playing through TSR adventures using 1970's and 1980's tools and you have the current state of part of the post OSR.
It isn't a good part, and while it can be fun to play old games and old adventures exactly as they were written (not as they were played obviously - that can't be done, especially if one has already been playing Pathfinder for six months), it has very little to do with an OSR that I think most of us who were once involved in it would recognize. In it's heyday on G+ the OSR often used B/X as a sort of lingua franca for running games in multiple and varied referee designed settings - but plenty of other games were played. Certainly they were discussed and borrowed from. Not only old games either, but story games, and the contemporary editions of the 2000's and 2010's.