Following the announcement of Brigitte's arrival on live servers, Brigitte will be available in Competitive play starting with Season 10, according to Overwatch developer Scott Mercer.
It takes a village to raise (and create) a hero!— Overwatch (@PlayOverwatch) March 15, 2018
Discover how lead character artist Ben Zhang and the team brought Overwatch's latest support to life.
Brigitte joins the fight: 3.20.18 pic.twitter.com/NJv7wLBEAR
His post on the Overwatch forums detailed a new rule that Blizzard has put in place regarding new heroes or maps and their addition to Competitive play.