Disposable baby diapers or even disposable adult diapers have no expiration date.
However the longer the diapers go unused and sit on a shelf the diapers then will likely fade in color and the tabs may stop being good at holding the diaper together.
The glue in the diapers that holds the diapers together as well as the tabs can start degrading and then the tabs may not be able to keep the diaper together without ripping apart.
Disposable diapers whether they are baby diapers or adult diapers are usually good on the shelf for 10 years or more before the glue starts breaking down.
However most diapers are used up way before 10 years anyway so it's not really much of an issue.
You can safely stock up on disposable diapers for your baby or for adults and they will last a long time.
I've bought old stock Pampers diapers before from an overstock store here and they were 10 years old but I got the diapers for $2.00 per pack so I figured it was worth it.
The diapers were just like new except the colors of the prints had faded just a bit but the diaper was still absorbent.
The diapers go into the trash anyway so it was not much of an issue.