A hero that wins enough retires from Classic Constructed and Blitz. Heroes earn Living Legend points for tournament wins, more for the biggest events and multiplied by how long they have been legal. LSS runs a check every Monday (United States time): a hero at 1,000 points or more becomes a Living Legend, and that hero and its signature weapon stop being legal from the Friday of that week. They stay playable in the Living Legend format, which exists for them.
The points per event type and the multipliers change between seasons, so they are not copied here: LSS's Living Legend page has the current table and the live leaderboard.