A few things taken from Mages that have been given to AD champions in the past few years.

Will of the Ancients was removed because, supposedly, healing from abilities was overpowered and difficult to balance. (read: "we didn't want to put in the effort to balance it for Swain and Vladimir") Enter . Almost exactly the same item. 80 AP -> AD (though 80 AD is more for most champions than 80 AP is, due to differences in scaling), 10% CDR, and the same amount of healing. But also has the bonus effects of working on Auto attacks and converting 30% of all damage taken to a bleed to give you time to heal. The cost is 1300 more, but it's also a better item in every way, including buildpath.

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Deathfire Grasp was too strong because end-game builds could have almost 200 more AP than is reasonably possible now, and adding 20% damage on top of that was outrageous since supports couldn't actually protect anyone.

Now we have Press the Attack, which has about the same effect, but causes them to take extra damage from ALL sources, not just magic, and caters to ADCs. And now, unlike before, Supports all have the option to shield and heal their allies. So Deathfire could be brought back, especially with this reduction in AP available, without much fuss.

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And finally, there's the simple reduction of AP available in the game. The items that have been nerfed/removed, when compared to those added/buffed, end up with around 200 AP missing from the game, accounting for the amount lost on Rabadons.

Meanwhile, the only AD items removed from the game were SotO (removed for similar reasons to Leviathan), Zephyr (because it didn't fit well into most builds, and now the increased AS on Triforce suits the three champions that built it more), Atma's (only ever worked on Garen), and Sword of the Divine (which was only AS, and has been replaced with the far stronger Lethal Tempo rune). Critical hit chance changed from something you might have 60% with an end game build to easily having 80% with all effective items. AD items have been buffed or unchanged, except for small decreases of maybe 10 AD here and there to account for larger buffs to similar items. New AD items have been added for fighters, ADCs, and casters-- the latter of which are often abused by ADCs.

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And with that 200 reduction in AP, we've also gotten an over 1000 increase in effective health in the form of Redemption, Locket, and other +%Healing and shielding passives on items that scale into the late game. So even a full-build mage can't kill the enemy ADC if the support isn't also CC'd.