I know I'm going to get a lot of people disagreeing with this, but personally I would rather not have Type: Null added to the game. It's my impression that its availability as a starter was probably an oversight (Sun/Moon have a lot of weirdness with having a large number of highly unique/special pokemon that technically aren't legendaries, so I can see a lot of potential for confusion!) that unfortunately didn't get caught in time to prevent it showing up in-game. However, I think that adding it to the game in general would risk disrupting both established plot precedent regarding legendaries and such (I should note that, while some legendaries are strictly shown to be one of a kind in the lore, others are explained more in a sense of "you'll only encounter it once in your lifetime" sense) as well as game balance in general. Without having Memories, Type: Null's ability is useless, but with them, it because extremely overpowered. Even without them, its signature move is essentially only a slightly weaker version of Judgment. Heck, everything about Type: Null is basically "look, scientists tried to make their own fake Arceus!" and it just doesn't sit well to me to make that available to player characters lightly.
Plus, Type: Null was also specifically designed, according to the games, to be a "beast-killer", an ultimate weapon against the Ultra Beasts. And while I'd love for the UBs to be a plot, I'm very concerned that having a bunch of Nulls running around risks unbalancing it.
As such, I think it's better to not add it to the game in general. Worst-case scenario, the one already in-game can stay with the understanding that it can't be passed on if the character drops and played off as a fluke. Assuming it was just an honest mistake, I'm not a fan of punishing a player for obtaining something legitimately at the time that in reality shouldn't have been offered. But I'm very much not a fan of the idea of it being available in the game at large, and feel it would cause more problems than it would solve to do so, including being a slap in the face continuity-wise to how very, very extremely rare and special legendary-type pokemon have always been treated by this game's plots in the past. For that to suddenly change will come across as ICly fishy, all other OOC considerations aside, and is likely to cause issues on that end as well.
So essentially I don't think it should be available, unless maybe in a weakened form (e.g. bootleg clones are bootlegs and not actually as powerful/special as the real thing) for the sake of preserving game balance.
Regarding Type: Null....
Plus, Type: Null was also specifically designed, according to the games, to be a "beast-killer", an ultimate weapon against the Ultra Beasts. And while I'd love for the UBs to be a plot, I'm very concerned that having a bunch of Nulls running around risks unbalancing it.
As such, I think it's better to not add it to the game in general. Worst-case scenario, the one already in-game can stay with the understanding that it can't be passed on if the character drops and played off as a fluke. Assuming it was just an honest mistake, I'm not a fan of punishing a player for obtaining something legitimately at the time that in reality shouldn't have been offered. But I'm very much not a fan of the idea of it being available in the game at large, and feel it would cause more problems than it would solve to do so, including being a slap in the face continuity-wise to how very, very extremely rare and special legendary-type pokemon have always been treated by this game's plots in the past. For that to suddenly change will come across as ICly fishy, all other OOC considerations aside, and is likely to cause issues on that end as well.
So essentially I don't think it should be available, unless maybe in a weakened form (e.g. bootleg clones are bootlegs and not actually as powerful/special as the real thing) for the sake of preserving game balance.