Why were AP Bruisers slowly phased out, anyway?
It's not that we never had any, but that the systems supporting ap bruisers were removed:
health nerfed/damage buffed.
health nerfed/damage buffed/added damage amp passive.
stats nerfed/level-up passive replaced by eternity passive (which is a sustain buff on spell spammers)
Abyssal scepter lost ap then was changed into
lost aura for stronger solo passive and gained eternity. Net loss in efficiency due to overlapping passives.
reworked into a support item from mr/ap/mp5 item for mages.
changed from tank item to mr/ap item; buff to ap bruisers.
lost 5 armor and cd increased by 30 seconds, which generally makes an armor/hp item or armor/cd item better for defensive purposes for a theoretical ap bruiser.
reworked and lost bruiser capabilities.
reworked and lost bruiser capabilities.
still able to have bruiserish builds.
better as bursty diver with current itemization and scalings.
no longer needs ap to deal damage.
post rework was changed to incentivize high ap comboing/deincentivize tank Gragas.
reworked in a way that cost him agency and power offensively, defensively, and to his utility.
still built as a mage rather than you'd expect as a juggernaut because of a combination of itemization and damage equaling greater sustain. (Correct me if I'm wrong, as he's such a rare pick and I never personally played him.)
has armor scaling and can forego ap now.
can still do enough magic damage to qualify, but struggles due to modern(ized) champions being able to outshine him in every role.
one of the few remaining qualifiers. Niche though.
It's an archetype I fell in love with by playing Sejuani originally. (I'm obviously not a fan of her rework.) It's also one that I always felt worked really well in the jungle, but items and champion design has moved away from this type of play slowly and started strongly favoring ad for bruiser builds. This is actually something I found kinda strange for one simple reason: If you remove strong auto attacks from the bruiser archetype (although with 
, this is out the window for ap too, but nonetheless, back in the day, before feral flare and such, it wasn't an issue), they're a lot easier to balance. The reason was simple. In order to have an ap bruiser, you needed 1 of 3 options: a damage over time spell like old
or
, a spammable low-to-medium damage spell like
(back when he was healthier and also had ap scalings on w), or auto-attack scalings like
. The first 2 are easier to balance for obvious reasons. A more healthy example than Diana would be
, though he is outshined by modern champs. The reason why the archetype could be healthy was actually pretty simplistic, when your autos and your spells have different scalings, you have less levers you need to control, which makes it easier to keep them from going from god to trash to godly once more.
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It is really annoying they nerfed ap bruiser items though.