It was in an interview in an article that was posted on GTA Forums, where he talked about GTA VI being difficult to write in a modern setting. Most of the stuff about GTA VI being an explicit period piece is rumors and speculation for now, but given this interview, it would make sense.
The original IGN article, for context
Dan Houser mentions how both SJW's and the Alt-Right have become more ridiculous in reality that any attempt satire would come off as ineffective.
Combined with a lot of the other rumors about the alleged "Project Americas" leaks and its supposed 1980's Florida and Caribbean setting and how GTA VI might be set in an HD Universe version of Vice City, I could see Houser wanting to make GTA VI a period piece.
As for the Shark Cards controversies, I've seen that pop up several times in both online discussions and in videos and articles about it, I'll have to find some specific ones first.