Soul Cube/Doom (2016)

The Soul Cube appears as a background object in both games set in the Doom 2016 timeline. It appears to simply be present as a Easter Egg reference to Doom 3: it cannot be picked up or used, and there is no storyline information associated with its presence.

Doom (2016)
The Soul Cube is present on a desk in Olivia Pierce's private office in the level Lazarus Labs.

Doom Eternal
The Soul Cube is found on the Slayer's main computer desk inside the Fortress of Doom in Doom Eternal. This is likely more of a nod to the fact that his counterpart in Doom 3 had the Soul Cube in his possession at the end of Doom 3 (& Doom RPG)]], and its now it's one of his 'souvenirs' he keeps in his room, memorabilia of past campaigns and battles.