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Regarding your question, when you disable online sign-in, it means that users can no longer sign in to Exchange Online using their credentials, which in turn means that third-party applications can no longer use OAuth 2.0 to authenticate and authorize access to resources.
Disabling user sign-in can impact application-level access because some applications use OAuth 2.0 client credentials grant flow to authenticate and are configured with application permissions, which by default enable such apps to access all mailboxes in an organization on Exchange Online.
You can find more information about how Exchange settings impact MS Graph access in the following documentation: https://learn.microsoft.com/en-us/graph/auth-limit-mailbox-access
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