--snip--
It's obviously wrong, Ordinals TX has size more than 0.
I didn't say Ordinals transaction size isn't zero. I said technically it isn't a waste in block space. Why? Because the data isn't embedded within the structure where the UTXO set is stored. It goes to the structure of the block where witness data are stored, which is prunable.
1. While it's true you can prune witness data, AFAIK there's no full node which let you do that.
2. It's still waste of block size since Ordinals TX has size more than 0. Possibility of pruning witness data actually means it's not waste of storage space.
The point is Ordinals embeds data in the structure of the blocks that store witness data, not the part of the block where the UTXO resides. In that context, I don't know what's "wasted" because none of the embedded data is "stealing" space from actual Bitcoin transactions. Plus as a consensus rule, the block size is regulated up to 4MB.
But Bitcoin Stamps is something different, and it should be more concerning in my opinion.
It's definitely more concerning, although it's easier to make such TX become non-standard.
Does making them non-standard come with a hard fork?