As far as I can tell, the DMG makes no mention of what happens to the contents of a portable hole or a bag of holding when one is placed inside the other — it simply states that the items themselves are instantly destroyed:
Placing a portable hole inside an extradimensional space created by a bag of holding, handy haversack, or similar item instantly destroys both items and opens a gate to the Astral Plane. The gate originates where the one item was placed inside the other. Any creature within 10 feet of the gate is sucked through it and deposited in a random location on the Astral Plane. The gate then closes. The gate is one-way only and can't be reopened.
Does this imply that the contents are similarly destroyed?
I’m wondering if this is a feasible (albeit a rather costly) method of permanently destroying a particularly tough foe — a medium-sized demon, say — who might happen to fit inside a portable hole?