If you purchase the physical version of the Call of Duty Infinite Warfare Legacy Editions on the Xbox One and PlayStation 4, you will need the disc to be inserted in the consoles to play the extra Call of Duty Modern Warfare Remastered game.
Published Activision confirmed the rumors about this situation in an updated FAQ post on the game:
Call of Duty: Modern Warfare Remastered is included on the Infinite Warfare Xbox One disc (no voucher code required) in the Legacy Edition and Legacy Pro Edition of Infinite Warfare. Before Modern Warfare Remastered can be played, an additional download is required. The game disc must be inserted to play Modern Warfare Remastered. Internet connection is required.
The PlayStation 4 version is slightly different in that Modern Warfare Remastered is a pure digital game, which will be downloaded via a voucher code or insert in the Infinite Warfare box. However, the Infinite Warfare disc must still be in the console to play Modern Warfare Remastered. The PC versions of Modern Warfare Remastered and Infinite Warfare are both digital downloads on Steam so no disc is required.
Call of Duty Modern Warfare Remastered, a graphical reboot of Call of Duty 4: Modern Warfare, will not be sold on its own, according to Activision. It is only available with the $79.99 Call of Duty Infinite Warfare Legacy Edition.