Announced at BlizzCon 2019 (see reveal panel), Blizzard was staying tight-lipped about the Diablo 4 release date but it will not release in 2021. In typical Blizzard fashion, they would not provide an exact release date as they stated the game is still in the early stages of development.
Latest Release Date update News 2022
Release Date Announced - 8 December 2022
Diablo 4 will release on 6 June 2023 as announced at The Game Awards on 8 December 2022.
The release date was announced with a show from Halsey and a new cinematic.
Dataming Microsoft - 7 December
Datamining Microsoft's store revealed a 5 June 2023 release date. 
Game Awards and Documents Rumor - 6 November
In a post on Windows Central they cite a source confirming a release date will be revealed 8 December 2022 at the Game Awards along with a new trailer and pre-orders. The release date that would be revealed at the show will be April 2023. 
Interview - 3 November
During an interview, Rod Ferdgusson stated that an open beta would not be until 2023 indicating a release date could happen spring 2023. 
During the Microsoft showcase in June 2022, Diablo 4 was listed as a 2023 release. It’s listed as the first tile in the 2023 section before Starfield which we know is coming out in the first half of next year.
The game could now be on track for an early 2023 release. The fact that Blizzard has already added an opt-in section for the Diablo 4 beta test indicates they could be ready to start test at the end of 2022.
A marketing leak  on 17 August indicated that the release date could be revealed at the 2022 Game Awards in December. This has not been confirmed by Blizzard.
Book of Lorath
On 1 August 2022, Amazon listed the Book of Lorath which could be an indicator of a release date window either in April 2023 or September/October 2023.
Release date details prior to 2022
Diablo 4 Release Delay - November 2021
During the Activision Blizzard third quarter financial call on 3 November 2021, Blizzard announced that Diablo 4’s release would be delayed.
While we are still planning to deliver a substantial amount of content from Blizzard next year, we are now planning for a later launch for Overwatch 2 and Diablo IV than originally envisaged.
These are two of the most eagerly anticipated titles in the industry, and our teams have made great strides towards completion in recent quarters. But we believe giving the teams some extra time to complete production and continue growing their creative resources to support the titles after launch will ensure that these releases delight and engage their communities for many years into the future.
These decisions will push out the financial uplift that we had expected to see next year. But we are confident that this is the right course of action for our people, our players, and the long-term success of our franchises.
Statements in 2019
Blizzard issued the following statement after the announcement in 2019:
We’re very excited to reveal one of our most eagerly anticipated projects and have you all take a closer look at what’s next for Diablo.
This comes with a caveat; we’re still a way out from launch. Because we’re heads down in development, we’re not launching a dedicated forum for Diablo IV yet.
However, we still want you to feel welcome to discuss the game amongst yourselves. While we may not be able to answer all your burning questions, we are committed to sharing updates with the community and participating when we can.
Beginning next year, we will be providing quarterly updates regarding the progress of Diablo 4’s development. This might take the form of a blog or a developer post here, which we’ll sticky and leave open for ongoing discussion until the next update is available.
We’ll also keep an updated table of contents of all updates in this post so you’ll have a convenient place to refer to them over the course of development. We’ve included all the news coming out of this weekend so far to get you started.
Thanks for patiently awaiting this next chapter, and we’ll see you in Hell!
Diablo 4 release date calculations
Although there were only three years between the release of Diablo 1 and Diablo 2 under Blizzard North’s guidance, there were twelve years between Diablo 2 and Diablo 3. Blizzard North closed down and development moved to Blizzard HQ in Irvine which delayed development.
Diablo 3 was announced in 2008 and took four more years of production before it was ready for release in 2012. The development of Diablo 3 started as early as 2001. Only two characters were revealed when Diablo 3 was announced, the Witch Doctor and the Barbarian.
Looking at the previous release date timelines, it would be reasonable to assume that Diablo 4 could be another three to four years away from release meaning the Diablo 4 release date could be in 2023.
There is one unexpected issue that has arisen during development in 2020, the global COVID-19 pandemic. Blizzard had to shift most development from working in the Irvine office to staff working from home.
It’s not known how much of an impact this has had on development in 2020. This could push the Diablo 4 release date back 6-12 months depending on the disruption. However, Blizzard did successfully pivot to a working from home model during the pandemic.
During the Activision Blizzard financial call in February 2021, they stated that Diablo 4 would not be released in 2021 which was to be expected.
The announcement of Diablo 2 Resurrected at BlizzCon in February 2021 should placate the fans somewhat while development continues on Diablo 4.
On 3 November 2021 they announced Diablo 4’s release had been delayed.
Was Blizzard ready to announce Diablo 4?
There has to be some speculation that perhaps Blizzard was not quite ready to announce Diablo 4 in 2019. Prior to the announcement in 2019, Blizzard had a tough BlizzCon in 2018 when they announced the mobile game Diablo: Immortal.
At the convention, fans were eager for a Diablo 4 announcement. The core fanbase was hugely disappointed with the mobile announcement. They were hoping for a full-blown game for PC and consoles.
Blizzard came under fire from fans during the BlizzCon 2018 Q&A. This prompted Blizzard developer Wyatt Cheng to say to the audience, “Do you guys not have phones?”. This response did not go down well and became a meme when mentioning Diablo: Immortal or even Diablo 4.
Even though fans were not expecting a Diablo 4 release date, they were hoping for some morsel of information on the next game.