Opened diapers do not expire.
Even sealed diapers do not expire.
Although overtime the diapers may start to degrade after several years it usually takes longer than 10 years for that to happen.
Disposable diapers are made of a plastic as well as SAP.
The SAP is basically good forever as long as it does not get moisture in it or soak up any water.
The plastic can degrade overtime but most often it takes over 10 years for that to happen.
I was given some Pampers Diapers that were sealed and open that were 10 years old and they were just fine.
I saw really no difference in the opened pack of Pampers Diapers and the Sealed pack of pampers diapers.
Diapers are not bad just because they are old and there's nothing wrong with using older disposable diapers.
Diapers really do not expire or get unsafe to use.
Just store the unused diapers in a dry non damp location and they can last for a long time.