trigger DeactivationDate on User (before update) {
    Map<Id, User> UserMap = trigger.oldMap;
   
    for(User U : trigger.new) {
        if(U.IsActive == false) {
            User Usr = UserMap.get(U.Id);
            if(Usr.IsActive == true) {
                U.Deactivation_Date__c = U.LastModifiedDate;
            }          
        }
    }
}