Diablo V runs on its own branch of the Diablo IV engine

Diablo IV had to start over technically: not the Diablo III foundation, a rendering engine modernised from the ground up. Diablo V does not. In a BlizzCon interview Windows Central published on 13 September, senior producer Tiffany Watt said:

We've had our own branch now for Diablo 5, for a while now, and we've been tooling that and making that specific to the game we're building [...] it means we can make it for our specific needs, not just what people want. We also don't want to carry over any tech debt or things we don't necessarily want in the game for the future.

So Diablo V is built on Diablo IV's technology, split off and shaped for this game. Some coverage calls that a "fork". Watt's word is "branch", and she gave no date for when it was made.

What Diablo IV launched without

In the same answer Watt addressed why Diablo IV shipped without features players had come to expect. Those were meant to be there at launch, she said, but the team was "frankly prioritizing some of those amazing creative system features" - and this time "it's really important we bring the whole package". As an example of what players call core, she quoted party finder and the armory.

That is not a feature list. Watt did not say Diablo V will launch with a party finder or an armory, and nothing about multiplayer has been published - see what Blizzard has said about multiplayer.

How it is filed

A developer statement. Windows Central interviewed Watt together with art director John Mueller and attributes some answers to "the team"; this one is Watt's, who says "As a producer" in the same answer.