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Are the Super Heavies, namely the Plague Reaper, legal as Lords of War?

Is this a thing gaming groups should decide on their own? I have seen the forgeworld and GW lists of legal lords of war (neither lists the plague reaper).
And in the case that it is not a legal lord of war, is this model (and so the entire Pandorax book) only legal in apocalypse?

After a bit of googling myself I saw that adepticon listed it as a legal lord of war, which is why I was thinking maybe its gaming group dependant?

The Warzone books are supplements for the Apocalypse rulebook. Without a note saying that they can be used in a normal game of 40K (whatever that is anymore ) then it's safe to assume any of the units presented in the book are for Apocalypse only.

As Ghaz said, the default is that only ForgeWorld units with the "40k approved" stamp are valid choices in a regular 40k game.

Beyond that it is pretty common for tournaments (such as Adepticon) to list specific LoW that are allowed.

If you aren't playing in a tournament then just ask your opponent if they would allow the model. Be sure to allow them to look over the rules so that they can make an informed decision. Some people will allow them because they want the challenge. Others might not if their army has no real counter to it.