Is there a way to check when the Idle User Deactivation happened?

Just thinking about approaches
1) We can't rely on system logs (login history/audit logs) because Appian only stores the last 30 days of login information.
2) I recommend creating a simple process that is called via a Web API on every user landing page load. This process records the current user’s timestamp each time they log in or visit. You then have full control to compare this stored last login time with your deactivation period and send notifications accordingly.

In agreement with Shubham, I went ahead and implemented my own system for this about 1 year ago, which tracks user logins manually per site/page (and does a nightly recap read-through of the login-audit CSV file and writes a dummy entry for any valid logins that may have been missed somehow), and a custom-designed process runs through and deactivates any users with no login for >60 days, writing various database entries for tracking purposes.

AFAIK one of the log files (user management) might reflect the appian-internal one, but I'm not sure.