Is the method trigger:old only used for the event "before delete"?

No you can't used the trigger:old for the event “before delete”

Thanks

Hi, Thanks a lot to clear my doubt.

In Before Insert Trigger.old collection is not able, it's null.
Trigger.old trigger is available in after update trigger and will hold old copy of current modifying record.
Trigger.old is available in Delete trigger .

Trigger.new is not available on before delete, and after delete triggers.

For future refrence:

trigger ParentTrigger on Parent__c (before delete) {
List<Id> parentIds = new List<Id>();
for(Parent__c p : Trigger.Old)
parentIds.add(p.Id);

Map<Id, List<Child__c>> childMap = new Map<Id, List<Child__c>>();
for (Child__c child : [SELECT id, Parent__c FROM Child__c WHERE Parent__c IN :parentIds]) {
if(!childMap.containsKey(child.Parent__c))
childMap.put(child.Parent__c, new List<Child__c>());
childMap.get(child.Parent__c).add(child);
}

for (Parent__c par : Trigger.Old) {
if (childMap.containsKey(child.Parent__c)) {
par.adderror('Cannot be deleted');
}
}
}

Thanks Shariq.