The term game mode might mean many things, but in this case, we'll focus on the difference between singleplayer and multiplayer game modes. Dead Keys will be single player, but there's nothing intrinsic about its design that indicates that it is for a single player only. It can be adapted to both local co-op multiplayer and internet-based multiplayer (using the Unity networking features). More information on Unity network, for the interested reader, can be found online at: https://docs.unity3d.com/Manual/UNet.html.
It's important to decide on this technical question very early in development, as it heavily impacts how the game is constructed and the features it supports.
