Blizzard has confirmed that Diablo 4 is still getting Set Items, though you'll likely be waiting until post-launch to use them in-game.
Game director Joe Shely explains during a recent roundtable interview that reimaging sets in a way that suits Diablo 4 properly wouldn't be possible for launch early next year, though the team is looking at including them as part of its live service.
"As we've been building the game, we felt like we could evolve sets in a new way relative to what we've seen in Diablo 3. But, we needed some more time to get them into the game in a way that would work well with the rest of Diablo 4, so you won't see sets at launch in the game," Shely explains. "We think sets are cool, and we want to do them right – so that's something we're looking at for our live service."
Blizzard initially confirmed that set items would be included in Diablo 4 way back at its reveal at BlizzCon 2019, though it reassured fans they wouldn't be as potent as they currently are in Diablo 3.
"There's a staff legendary in the demo that turns [the spell] fireball into a triple fireball spell," former game director Luis Barriga told Polygon back in 2019. "The difference might be that, you know, a set right now in live Diablo 3 gives you bonuses more in the thousands. I think we're designing the game right now to be a little bit more constrained to avoid getting us into that super escalating power curve."
If you're unfamiliar, Set Items are essentially strong pieces of gear that offer greater power when fully put together. They're 'set' as they always provide the same bonuses, and, well, they come as part of a set. They were initially introduced in Diablo 2, though it was 3 that they became truly dominant. It remains to be seen how they will affect the Diablo 4 meta should they arrive, though we have no doubt their influence on the Sanctuary's unfurling fashion scene will be greatly felt.
The Diablo 4 beta hasn't convinced me yet, but I can't wait to play more (Set Items or not).