I caught sight of some comments on YouTube videos associating the sprites used with either Spirits of Hell (S3 sprites in the nightmare and cutscene pre-Test Level) or the Blood Scream games (Mod.Gen sprites that are currently in use).

Just to make it clear, the S3 sprites used in Test Level's intro in Arcade Mode are just there to be about regular Sonic.exe prior to the character falling in the digital pit.

In addition, neither Spirits of Hell nor Blood Scream are connected to the continuity related to Project Eclipse, both games of which remain in their own separate universes. This game instead branches off of the ORIGINAL Sonic.exe game just about everyone's familiar with nowadays. Please do not solely associate the used sprites with those Sonic.exe games. I really don't think the original creators of said sprites would really appreciate it.