Comic fans will recognize this as the Ronin costume, but why has Clint Barton -- who's been absent from the movies since 2016's Captain America: Civil War -- adopted this identity in the 22nd movie of the Marvel Cinematic Universe?
Clint was killed during the Avengers Disassembled storyline and resurrected during the reality warping House of M, but was (understandably) psychologically damaged by this experience. He abandoned the Hawkeye identity and rejoined the team as Ronin in New Avengers No. 27, on a mission to rescue a captive Echo in Japan.
Hopefully the Ronin subplot in Avengers: Endgame will be a little less convoluted, but it's likely that emotional trauma and/or avoiding trouble for escaping house arrest will be behind Clint's decision to take on a new identity.