Personally, I think a lot of the BroSR (BrOSR?) thing is a mix of reactionary attitudes and likely some unfortunate realities baked into the pie that probably weren't really addressed whatsoever in the first 40 or so years of the game and have become political (and thus polarizing) because of the groups that are pointing a lot of this stuff out. Particularly the LGBTQ+, women, and non-white gamers that have really embraced TTRPGs since 5E blew up.
I don't think a lot of us cis- white- kids really thought too much about what was in the games from a non cis-, non-het, non-white perspective in the 70s, 80s, and 90s. Hell, the controversial part during the time was supposed links to Satanism, not the hard-not-to-see-now parallels of racism and treatment of indigenous peoples.
Looking back, it starts to seem like there really is more to the 'original sin' narrative regarding a lot of the basis of TTRPGs than just being products of their time and the problematic stuff being more happenstance, reinterpretations of original intent, or simply misguided, but well-meaning work (1E OA fits this last bucket pretty well). You read those key works in Appendix N and you can't help but notice that many were particularly racist. We now know that M.A.R. Barker (Empire of the Petal Throne) was a particularly avowed antisemite and Nazi sympathizer. The quasi-infatuation with Nazi German military stuff that was just the norm in the 1970s wargaming communities that birthed D&D. Fascism, white supremacy, and heteronormative culture doesn't seem to have been overtly built into the games and then the gamers, but there was definitely tacit acceptance of a lot of stuff that probably more intentional than we like to admit.
The BrOSR stuff, at least to me really has a lot less to do with that stuff that is built in but is actually just another pathway for far-right actors to groom and engage with a group of disaffected, mostly white, men. It's an offshoot of the same playbook that came out of Gamergate in the video game space and is really just a part of the wider "manosphere". To provide an historical analogy, the KKK, specifically the large, national group that developed in the 1920s has really little connection to the historic one of Reconstruction. The latter group simply picked up the trappings, expanded the scope of the previous group's ire, and ran with it. I think we see the same thing today.
Basically, the BrOSR folks look for minor grievances in the TTRPG community, especially where it takes a bit of understanding intersectionality or willing self-reflection to work out, says, "these people are trying to inject politics into your game" which creates sympathies, as no one really likes to feel that they were engaging in something to cause pain to a group. The door is opened, and then more and more gets pumped in. I watched this happen personally with my EVE Online corp 10 years ago. It is a very well-honed tactic and it works.