There's no way to view this in-game as it has been deleted, and probably will not be added back into the game in the near future, as it has been removed from the game either partially or entirely.
The Underground Storage was an area next to the A Bizarre Building, where there is an underground building which leads to the a black back door, leading to the Underground Storage room.
To enter the storage room, the player needs to have a stand that could teleport through walls. The storage room also replaces the Tom's Juice Shop, which has been deleted.
This Storage is also near the sewers, which is also located in the Underground space. As of today, there is no explanation as to why this room exist, as there is nothing unique included in this room. The only explanation could be for aesthetic purposes, with boxes and a painting between them.
The door that leads to the Storage room.
A bigger perspective of the storage room, however there is not much there.