Will they need to even nerf Regigigas?
Even with full stats (Not 9% nerfed) it doesn’t best the Fighting department (Because Rock Smash and Focus Blast are just that bad) and while it does with Normal, (With Porygon-Z just behind it as runner up with Normal type moves) that type isn’t really needed as it doesn’t hit anything for SE damage, and it isn’t any better than the big dragons (Rayquaza, Palkia, Dialga, Garchomp) except for in partly cloudy weather.
Answers
Given it's terrible movepool, Niantic didn't need to nerf Ho-Oh either, seeing as how it still wouldn't be able to compete for top fire DPS even without the 9% stat nerf.
So far, they've done the 9% stat squish for every Mon that would have a CP of over 4000, so I don't imagine Regigigas would be excempt from that.
I don't think they would have bothered adding all 4 of Deoxys's forms to the game master if only one was to be made available.
Also, none of Deoxys's forms break 4k CP so none of them will face the nerf. I brought up Giratina specifically since the forms (in the main series anyway) are interchangeable. With one form being above their nerf threshold and the other being below it it'll be interesting to see how it's handled. I'll be a little disappointed if they play it safe by nerfing both forms.
We dont know that its move set will be, I actually think the rocksmash/blast set is because you will need the three lesser regi's to fight Gigas, it has a signature move, Crush grip, and they could give it a good normal fast move, assumptions as to what one of the most ambiguous pokemon will be wont really get anywhere
I highly doubt Niantic is going to require us to use the other 3 Regis to fight Regigigas. That would be foolish on their part (given that people typically haven't been raiding Regice/Registeel hard, and thus don't have good candidates to power up), and would result in fewer people being able to do Regigigas and extensively long raid battles for those who could. This would be a very harsh requirement that would not at all be friendly to the playerbase.
Also, since none of the legendaries have been released with their signature moves, I imagine Regigigas won't either.