I feel like there is alot of mis-information going about on how the AP items are getting nerfed.
EDIT: Turns out a bunch of the tanky items are getting their numbers changed aswell, so some of the stuff i have posted here is wrong as I had not seen the changes. However the point still stands that AP items are not getting nerfed.
At first glance, things might appear that way, but you need to look at the whole picture to get a better idea on what's going on. I feel like since alot of the AD items are getting reworked/changed it's muddying the water and people dont get what's happening.
Lets look at the some examples;
Firstly, deathcap. Currently, ignoring any extra AP you get from other sources, the item alone gives 162 AP at a cost of 3500. This gives us a gold efficiency of 100.67%.
162*21.75/3500=1.0067... (21.75 is gold per stat for AP derived from amp tome).
The new deathcap will have it's price as 3800 so the new efficiency will be;
162*21.75/3800= 0.9272... 97.72% efficient.
100.67% to 97.72%. A drop of about 3% So that's bad right? Well before me make any conclusions lets look the changes to IE aswell.
Old IE had a gold efficiency of 102.11% (without it's passive). The new one will give 15 less AD but will be 300 gold cheaper. So the new efficieny will now be 95.43%. This is a drop of about 6.5%
We could do the same for the new Essence Reaver; Old Efficiency =117.08%, new efficiency would be 104.46% a drop of about 12.5%
And this happening across the board, not just with AP items but with AD items too. Blood Thirster, Ravenous Hydra, Trinity Force, Youmoo's Ghostblade... etc etc
And this is just comparing the current season to the new one. With the prices changes to long sword and brawlers glove, AD and Crit will change the gold efficiencies of both of these stats. This means that AD and crit will be cheaper to get, but ultimately lowers the slot efficiencies of the premium tier items.
So what does this ultimately mean? Well it means overall this will be a buff for those buying defense, since those stats aren't getting messed with.
That said, the new Last Whisper looks like it might be very strong. 1300 gold for 25 ad and 30% armour pen on bonus armour. The AD will be worth 825 gold, and so the armour pen costs 425 gold. This means the enemy will only need to have 21.25 bonus armour for Last whisper to become gold efficient. And this only gets better the more armour the enemy has. This is without even considering Last whisper's new upgrades.
However, last whisper won't do much stuff agianst stuff with no bonus armour. So it's actually all quite clever. AP will have a slightly harder time with tanks, but will still be doing damage as much damage as their taking compared to other damage dealers. AD champs can build the new last whisper to deal with tanks a lot easier, but this will hurt their damage against other squishy targets.
TLDR: All offensive items are getting nerfed, except last whisper, which is only good against tanks now