Indeed, important to mention OP_RETURN, but iirc Stamps followed the example of Counterparty and did not use it, specifically to avoid being pruned (though the history of this is complex and controversial).
Discussion
Yes, they do use bare p2ms, which is not pruned. But the last time I've checked, the ordinals was the actual problem, because people crate a ton of small amount (10k sats) UTXOS to "hold" inscriptions.
If you roll the numbers, small size p2tr dominates this recent surge in UTXOs
Right, i see. I guess what I'm saying in the OP applies mostly to them.