There are some NPCs who are called X Shadowmage, so that works.
I find Shadowmancer looks too pretentious, but that’s just me.
Everyone loves an example: https://classic.wowhead.com/npc=4813/twilight-shadowmage
Pyro and hydromancer flow better than shadow or voidmancer, which is why I think I like it less. But it still works.
So you have options! Go and shadow it up!
The Affliction warlock weapon was originally wielded by an Eredar, who was described to be the “first necrolyte”.
Affliction warlocks use shadow magic and souls to cast magic normally powered by decay. Many affliction spells are death-themed rather than obvious shadow- or fel magic: drain soul, siphon life, deathbolt, vile taint , unstable affliction. Corruption may be counted among this too and even shadow bolt is in some sources described to be death magic. While they can’t raise the dead due to in-game mechanics, death magic is more than just raising a corpse.
AU Shadowmoon orcs are the perfect example of affliction warlocks in-game. They either outright use shadow magic or use the Void/souls/both to perform necromantic feats and other kinds of death magic. Shadowmoon Bone-Menders use Shadow Word: Frailty to turn the enemy’s bones brittle (ouch btw? that sounds nasty af).
“But affliction warlocks still can’t summon undead!” While that sucks, fear not: the Shadowmoon are seen using void walkers in addition to Void-powered undead.
Also, note that necrolytes are typically capable of raising skeletons at best, which generally fits with what we see of undead raised through the Void: it can animate bones or fill up a corpse with void energy to make it walk again (see the void elves and their necromantic shenanigans involving the ravasaur), but that generally seems to be the end of its potential to create undead.
Finally, to top it all off, there are even npcs named Shadowmoon Necrolytes who exclusively use shadow/void spells.
Affliction warlocks aren’t perfect representations of necrolytes, true, but there are many, many similarities, so I rest my case.
Shadow mage would be the word, it’s also a rpg class, and in the rpg invented the shadowbolt.
There is also shadowcaster as a npc title that’s not too uncommon.
And I think there is shadowweaver.
TBC gave us nethermancer and there is the astromancer solarian, however she might have ended up shadow casting the same way star Augur does in nighthold.
And as said shadowmancer doesn’t sound right, I’d wager due to the Latin/Greek suffix added on a English word. It’d be umbramancer I’d think then, or nihilmancer.
While your overall post has a point, I feel compelled to nitpick here: Drain Soul and Siphon Life are both almost definitely Fel. Drain Soul has long been the spell used by warlocks to produce soul shards, and we know the Legion uses souls to power a lot of their machines. Likewise, Fel can be created by draining a creature’s life force, so it stands to reason Siphon Life falls under this category too.
You could likely stretch it further and argue that most, if not all, spells that in some way generate or use Soul Shards have to be Fel-based, but I think that’s being a little too pedantic. DS and SL are just the main two of those you listed.
