This question was asked in 2014 on the Unreal Engine forums. Tim Sweeney wrote: https://forums.unrealengine.com/showthread.php?1515-Older-Engine-sources&p=8748&

This question was asked in 2014 on the Unreal Engine forums. Tim Sweeney wrote: https://forums.unrealengine.com/showthread.php?1515-Older-Engine-sources&p=8748&viewfull=1#post8748 https://forums.unrealengine.com/showthread.php?1515-Older-En... >There is a long history on this topic, but the short summary is: We've looked at this extensively and concluded that releasing UE2 & UE3 source would be impractical due to the large number of integrated third-party libraries which have limitations on redistribution. The public source code release only became practical with Unreal Engine 4, due to an extensive effort to clean up external dependencies. >UE1 might be possible at some point in the longer-term future, but that would be more for nostalgia than practical utility.