Yes, sadly, it is a 'thing' and has been for a long time. But I believe today it is mostly fantasy - akin perhaps to the fantasy of being bit by a vampire and being turned into one.
I was accidentally infected over 25 years ago (torn condom with my late long-term partner). The very last thing I would ever want to do would be to pass my infection onto someone else.
I have always been open about being HIV+ with potential partners, and I would only have sex with guys who were also already HIV+/"poz". My hook-up profiles have always included that I was HIV+ and only interested in other poz guys. But I had to also add that I was not interested in "bug-chasers" (or anyone who was PnP). Bug-chasers still contact me.
When I read their posts, it is really clear that they get off on the idea of being infected - it is their fetish. But, I think the vast majority of these guys are already HIV+ but they want to re-enact and get off again and again with their fantasy.
II have a hard time coping with them - their fantasy is an absolute turn off for me. I buried my late partner, have had three AIDS-defining illnesses and was forced to end an incredibly successful international career because of HIV. I am very lucky to be alive today, and have lived and continue to live an adventuresome, fulfilling life but I know the facts about HIV and it is the farthest form a fantasy as I could imagine.
Today, I am somewhat open to playing with someone who is on PrEP (I am undetectable so am already unlikely to transmit HIV). But it has only happened once, and it is a bit of a challenge after decades of the freedom of playing only with guys who were already poz.