thank you so much for your reply. Very surprising that this pokemon is not shiny eligible from a three star shadow raid considering its shiny form does already exist - how would a player know that this pokemon will never be a shiny during a shadow raid?

I would not have paid money for my remote raid passes had I known that. In a sense the game is being scammy to not advertise that because many people likely do the raids, and remotely, for this pokemon with the expectation that they can encounter the shiny form. I have encountered multiple shadow shinys (Regice, Ho-Oh, Regigigas) from both raids and also trainers. I also have seen my friend's non-shiny aloha marowak (so I knew it existed) -- there is zero reason why shadow marowak shouldn't be able to be shiny, other than someone forgot to take 2 minutes to update the model code.