With the latest update to the Shadowlands Beta today, players are now able to start testing addons. This will help give addon developers a chance to test changes to their addons and make improvements with plenty of time to fix bugs before Shadowlands launches in the fall.
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With the latest build of the Shadowlands Beta, addons are now enabled for use.
Beta testers who have been in-game will need to exit the game and update their client to the latest version (build 35213) in order to test addons.