You're an Org Owner of a Slack Enterprise Grid org.

According to Slack's official documentation on Enterprise Mobility Management (EMM):

"When EMM is required, any users who are not on an approved device and have not completed setup will be immediately signed out of Slack on their mobile devices."

There is no grace period of 24 hours, 72 hours, or one week — once EMM enforcement is enabled, users must be using an approved device immediately or they are signed out.

(Reference: Slack Administration Study Guide - Mobile Security and Enterprise Mobility Management)

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