Oh, HPPD is definitely a thing, but extraordinarily rare.
I may have misspoke about the brown acid - this was a legit warning resulting from “home-brew chemists” attempting to make their own LSD and failing to create it properly. Most of the supplies back then were direct from Sandoz (Novartis) and basically were being given away to the scientific community for novel testing. Fun stuff.
I’m talking about the hyperbole of “acid flashbacks” which was a narrative introduced to discourage and demonize LSD usage by the political and intellectual opponents of the Nixon administration. “Rots your brain permanently” and all that other garbage.
Turns out regular LSD usage by the “hippie” community and by many people involved in high-level education (particularly college and university professors) was making people feel more connected and empathetic towards one another, and that just didn’t do for the Republicans who needed everyone to fear “the other”.
What they also did with marijuana and heroin, and subsequently with crack cocaine, was truly abhorrent.