https://www.cnet.com/news/hawkeye-lives-avengers-endgame-trailer-confirms-ronin-fan-theory/

This costume first showed up in 2005's New Avengers No. 11.

It's possible that he lost his family -- whom we met in Avengers: Age of Ultron back in 2015 -- in Thanos' deadly snap. He and wife Laura (Linda Cardellini) had three children, and losing any (or all) of them may have left him traumatized and seeking vengeance.