You do NOT need Xbox Live to play IL-2 Sturmovik: Battle of Stalingrad offline. In fact, it's a PC game, not an Xbox title.
IL-2 Sturmovik: Battle of Stalingrad (2013) is a Windows/macOS/Linux game published by 1C Game Studios. It has never been on Xbox. The "Xbox Live" reference in the question is a misunderstanding.
Enjoy the full sim offline—no subscription required.