The reason for Clint Barton’s absence in Avengers: Infinity War is explained away in one line during the events of Avengers: Infinity War.
That’s where Clint has been since taking on Team Iron Man in that awesome airport battle, and he’s been resting up to help the original hero lineup from The Avengers round out the current slate of Marvel films in one more on-screen adventure next year.
As noted by Black Widow before the Battle of Wakanda, both Clint and Ant-Man hero Scott Lang were given plea deals after the events of Civil War so they could return to their families instead of taking prolonged stints on The Raft prison.