Call of Duty: Infinite Warfare's special editions came with Call of Duty: Modern Warfare Remastered as an added bonus, a remastered version of 2007's Call of Duty 4: Modern Warfare. Ever since the fans have been wanting a standalone release of the remastered classic.
Amazon Japan has listed Call of Duty: Modern Warfare Remastered for standalone release on PS4 on June 20th, which would likely mean that the Xbox One release would happen a month later.
Earlier this same date (June 20th) has been used by US retailer GameFly and by two German retailers.
Activision has yet to confirm that Call of Duty: Modern Warfare Remastered will get a standalone release. This means that such information should be treated as rumours at this point in time.
Thanks, Charlie Intel.