Newey's exit comes after a turbulent few months for the Milton Keynes outfit, and controversy surrounding Horner.
The design ace said at the Miami Grand Prix that he was 'tired', and Horner insisted that the pair will remain friends after their working relationships ends.
Horner reveals decreased Newey 'access'
Though there is not yet confirmation of Newey's next move, Ferrari have been strongly linked as a likely destination.
Horner has said that Red Bull have restricted Newey's access to information and what activities he takes part in as his exit nears.
"In Miami, Newey did not participate in any briefings over the weekend except for the one on strategy," said Horner. "He does not have access to any data and is not designing any pieces.
"He is now focused on the RB17 until the end of his contract," he added.
“Between now and the end of the year he will participate in some races, mainly those where we have RB17 customers."
The RB17 is Red Bull's hypercar project, on which Newey has worked as lead designer. It will be his final car with Red Bull.
The team recently announced that it would be unveiled at this year's Goodwood Festival of Speed.