VADP is not supported with vSphere 6.0. You can continue to backup using VADP with NetWorker 9.0 as well, provided you don't upgrade the ESX and vCenter.
As far as I know, it might be supported with the next version of NetWorker 8.2.x. Please check once it is released.
VADP is being deprecated with vSphere 6. This isn't a decision from the NetWorker side to call VADP legacy. The APIs provided by VMware for VADP backups still exist with vSphere 6, but they're being badged as Legacy in favour of VMware's own vSphere Data Protection (VDP) appliance. That's what NetWorker integrates with under vSphere 6. That's unlikely to change moving forward and any additional support for this won't be initiated from the NetWorker side.
Plain and simple it's better than the original VADP. If you're planning a move to vSphere 6, plan the change to the backup methodology as part of the upgrade.
the answer of the EMC Customer Service:
Networker 8.2.3 which is expected to be released end of march/16 will support this version of vCenter server and also vCenter v6.0.