Is it possible to automatically send email to someone after review's overall status is %100 completed?
No, there is no out of the box solution to send an email on completing the review to 100%.
It might be possible to achieve the same using custom review escalation workflows. May be with an iterative loop that frequently keeps checking for status of review till it gets changed to 100% and then sends an email.
Note: There is an email template 'ReviewCompletedEvent' using which an email is sent when state of the review changes to 'Completed'
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