As you can see in the quotation marks I mentioned the WoW releases(Chains of Domination and TBC Classic) might be vice versa, etc., or may release later depending on their current beta testing. I ended up getting one right so far with actually TBC Classic releasing earlier than my mid June prediction. Vice versa puts Chains of Domination release at mid June since end of May is so close to the June 1st release of TBC Classic. If D2: R doesn’t release on June 29th, 2021, and releases on my other guess of August 10th, 2021, then Chains of Domination will probably release early July!
It will probably be about 12-14 days long since it will be for Multiplayer, and stress testing the servers. Also, the other classes will be available, and probably Acts 3-5 as well.
Diablo® II: Resurrected™ Technical Alpha — Experience the Legend
Q: Why are there only three classes playable in the Technical Alpha?
A: Diablo II: Resurrected will launch with seven playable classes—the Amazon, Barbarian, Sorceress, as well as the Assassin, Necromancer, Druid and Paladin. Currently, we are focusing our gameplay testing on these three classes. We will be including other classes in the upcoming testing phase.
Q: Why are Acts 1 & II only playable in the Technical Alpha?
A: Diablo II: Resurrected will launch with the full narrative experience of the game, alongside the narrative content of the expansion, Lord of Destruction. For this test, we really want to hone in on early gameplay in the settings of Acts I & II.
The Devs have been working on D2: R since 2019. Most of the game is finished. There is a reason the Alpha test was only 4-5 days long, and the Beta test will probably be about 2 weeks long so people can play all 7 classes, and all 5 Acts to get a feel for the whole game, and find any more bugs/issues/etc. Currently they are fixing issues that were presented in the Alpha test leading up to the Beta. Once Beta begins, and ends they will work on any more feedback from the players, and then the game will be ready to release after that. We’re looking at a June 29th(at earliest) to August 10th(at latest) release of D2: R! 
June 29th, 2021 is 2 days before the start of the 2nd half of the year so it’s still possible since it is roughly close enough. This would be the earliest it could possibly release, though. The latest release would be around early-mid August(around August 10th, 2021)!