‘after undelete’ trigger is available in Salesforce but for limited set of objects.
Following is the list of objects on which after undelete will work :
Account
Asset
Campaign
Case
Contact
ContentDocument
Contract
Custom objects
Event
Lead
Opportunity
Product
Solution
Task
The after undelete trigger events only run on top object. For example, if you delete an Account, an Opportunity may also be deleted. When you recover the Account from the Recycle Bin, the Opportunity is also recovered. If there is an after undelete trigger associated with both the Account and the Opportunity, only the Account after undelete trigger event executes.