The beta test, apparently, will be launched in the very near future.
The Diablo 2 Resurrected remaster will be released on September 23, 2021. The game’s release date was announced at the Xbox launch event during E3; simultaneously, the developers released a trailer showing the main features of the game.
Before the release, it is planned to conduct beta testing of the multiplayer part of the remaster. Blizzard has not yet specified when testing will start; it can be accessed by pre-ordering.