You say that is if it's somehow true and not just made up fantasy nonsense.
I guess what I mean is that when it comes to the tropes of the D&D Wizard, it's very much in the intellectual and categorical, almost proto-scientific space. So Wizards would find it important to split between the disciplines.
Meanwhile, there already IS a Sorcerer subclass that combines Nethermancy and Necromancy -- the Shadow Magic Sorcerer - and they draw specifically magicl from the Shadowfell. Sorcerers are more impacted by the origin of the magic that is behind the spell; Wizards are more about how can be write/draw/say/formulize spells to bring forth magic? A Shadowfell Wizard is very possible to make, I'd just argue it's not something that belongs in the official core WotC material - at least not before a bunch of other concepts.
I do think one could argue for dual-discipline schools, where Wizards are drawing from both Illusion and Necromancy, for example. They've done this once with the War Magic Tradition, which draws from both Abjuration and Evocation. I honestly would love a Beguiller a la the 3.5e PHB2 that is a Wizard tradition specifically focused on combining Illusion and Enchantment. But if a Wizard is doing that, they're combining specific spell schools. Nethermancy, like Pyromancy, were REALLY WEIRD choices for spell schools that were sort of square peg round hole tropes. Pyromancy itself was only in a
Dragon magazine article, if I recall. Nethermancy was in Heroes of Shadow, and was trying to force the Shadowcaster concept from 3.5e
Tome of Magic into the Wizard. This is something WotC were doing in general during
D&D Essentials - somehow Witches and Sha'ir also got crammed in as Wizard subclasses despite sharing more with Druids, Sorcerers, and Warlocks (Witches) and just plain Warlocks (Sha'ir), narratively. At least the 3.5e Binder got to fit into the class that made sense for it - the Warlock!
Anyway, I was mostly making a off-handed comment as I was reading through the thread about what I though regarding the concept of combining these and tying a Wizard to the Shadowfell. It just felt more Sorcerer to me, and of course we have that in Sorcerer already. But of course if you want it in your game you can make it happen!