1. Create a Custom Field on the User object to store the user deactivate date time.

2. Use the following sample trigger on the User object.

Sample Code:

trigger UserTrigger on User (
    before update
) {

    Map < Id, User> UserMap = trigger.oldMap;
   
    for ( 
        User objUser : trigger.new
    ) {
    
        if ( 
            trigger.oldMap.get( 
                objUser.Id 
            ).IsActive &&
            !objUser.IsActive
        ) {
        
            objUser.Deactivation_Date_Time__c = System.now();
                    
        } else if ( 
            !trigger.oldMap.get( 
                objUser.Id 
            ).IsActive &&
            objUser.IsActive
        ) {
        
            objUser.Deactivation_Date_Time__c = null;
        
        }
        
    }
    
}