Data tag ArmorItems or HandItems not working on /summon or /entitydata, but other data tags do. I checked this from /entitydata's information from a command block, and I saw both HandItems and ArmorItems in there, so I believe theyre still active data tags. I tested this on two seperate worlds. I also tested this on 1.10, and the data tags worked there.

Edit: Works with Count tag:
{ArmorItems:[0:

,1:{},2:{},3:{}]}