Riot, Vel'koz support feels horrible now
Being reliant on AP makes him incredibly weak as a support. You used to be able to rush sorc boots and do respectable damage, but now that a large chunk of his power budget is true damage which scales on AP, this is no longer an option.
Part of his power budget going into mana sustain for killing enemies with his Q is only good for lane vel, and even then I don't think anyone wanted it to begin with. Vel's W and E now barely do any damage and are only there to set up for his passive. This means you have to put yourself into danger for longer to get the same amount of damage as before. This is basically a death sentence with all of the mobile champs that are currently being played.
To top it off, his ult feels terrible. Since you can't rush magic pen, it does way less damage than before, and since you can't reliably land the passive (mobile champs), getting the true damage off is highly unlikely. Even if you do get it off, it still doesn't do as much as before. Velkoz players almost always got lots of magic pen, so the ult was more or less doing true damage anyway, with the added bonus of applying passive stacks. Also, since the ult no longer applies stacks, you can't use it to burst people. you used to be able to ult a low-health enemy just long enough to get three stack and kill them, but now that this is no longer the case, they can just walk into FoW, flash over a wall, flash into you and out-dps and out-lifesteal your ult because adcs, etc. and be perfectly fine, since they have extra time to do something about it.
There's honestly no reason to play vel'koz support anymore. There are better supports which do more damage, have better cc, and synergize better with the new mage items. Thanks to his heavy reliance on AP, he is more or less forced into a laning role.