Activision Blizzard, during a recent earnings call, provided a quick update on Diablo 4, and possibly hinted at its release date. According to Activision, the game's development is making "strong progress," which is a bit vague, but it's coupled with word that the team is dedicating resources to creating more in-game content for the long term. In other words, the team is dumping resources into replayability and post-campaign content to ensure that in the modern games-as-a-service market, it retains players, which is smart, and not much different than how it approached Diablo 3, which continues to be supported to this day, although not substantially.
Activision Blizzard didn't have much else to say about the game, but they did seem to hint it could be released in the next 18 months, likely in the 2022 part of these 18 months.
"Blizzard's launch of Burning Crusade Classic in June marked the start of what is intended to be a very significant 18-month period for content releases," said Activision Blizzard during the earnings call.
A "very significant 18-month period for content releases" surely includes at least one of Blizzard's major upcoming releases, which is to say, either Diablo 4 or Overwatch 2 or, of course, both. If it's one or the other, it's likely Overwatch 2, as we've seen more of it and there's more indication it's further along in development. The way Blizzard talks about Diablo 4 suggests it's more of a 2023 or later game, but it's possible it's 2022-bound, especially with a comment like the one above.
Unfortunately, this is the limit of the update, and right now, there's no word of when we will get another update, let alone a proper update.
Diablo 4 is in development for PC, PS4, and Xbox One. At the moment of publishing, there's been no word of PS5 and Xbox Series X|S versions, but it's safe to assume these are also in the pipeline, especially if the game is a little ways off.