It appears that GameStop Italy might have leaked the release date of Destiny 2 ahead of the planned announcement at E3 2017. Several users on NeoGAF have started spreading the image above showing that it will release on September 8. This does make sense as Month of Mayhem Strikes for the upcoming Destiny Age of Triumph event would end around that time.
Please keep in mind that Activision nor Bungie have confirmed this release date. Destiny 2 is in development for PC, Xbox One, and Playstation 4.