Per the IRS:
The IRS cannot cancel your EIN. Once an EIN has been assigned to a business entity, it becomes the permanent Federal taxpayer identification number for that entity. Regardless of whether the EIN is ever used to file Federal tax returns, the EIN is never reused or reassigned to another business entity. The EIN will still belong to the business entity and can be used at a later date, should the need arise.
You (well, actually the plan sponsor) can always call the IRS to inquire that the EIN is still active: (800) 829-4933.