The Elite
BOSS
Characters announced in press releases are unquestionably not spoilers.
I don't recall there being a press release stating that Josh Brolin as Thanos was in GotG. Do you have a link to the actual press release? I know I knew about it but that's because I'm pretty lax about spoilers. I must have just missed the press release itself.
I recall there being an announcement that he would be playing Thanos in the Marvel Cinematic Universe thought. That was quite clear and his appearance at Comic-Con was all about that. There was a lot of hints and speculation that he would be making an appearance in GotG but nothing concrete regarding the size of his role. I'm not surprised people didn't know about it. They never went out of their way to make it common knowledge.
I'm with Sculli on this.