According to a Steam post by Larian, the Patch 8 stress test build is scheduled to launch in January. Players will be able to access it through Steam on PC and Xbox and PlayStation on consoles. Players on Mac and GOG are not able to access it. The interest registration form is currently active.
Before Patch 8 is released, Larian stated that it intends to "rigorously" test it for instability or gameplay problems.
“With your help, we’ll be able to keep an eye on any funny business,” the developer said. Part of this is testing crossplay. "Bringing crossplay to a game the size of Baldur’s Gate 3 has been no easy task," Larian explained, "and we want your help to test-drive this new functionality.
“Share the stress test registration link to summon your friends and settle in for a crossplay campaign, or head to the Larian Studios Discord Server to look for a group to play with."
While Patch 8 is Baldur's Gate 3's last major update, Larian has promised to continue to support the game's modders, with “big updates still to come,” including more functionality “to let you tell your own stories.” Since the official mod tools were made available in September, gamers have downloaded over 70 million modules and uploaded over 3,000 mods.