Bronze league confirmed is an internet meme that originated from the Starcraft 2: Wings of Liberty GameFAQs message board. It's basically said when a user makes a complaint about something in the game that only applies to them because they are in a lower league, such as balance issue or a strategy being cheesy.
User 1: It's starcraft people, not rushcraft, where you to play to win in 5 minutes.
User 2: bronze league confirmed, nothing to see here
User 1: Void rays are so OP, they need a nerf. When they rush with them there's no way to stop them
User 2: bronze league confirmed, Void rays are easily countered.