Rich Nost on Nostr: I'm assuming he means the witness data part of the transaction, which is where ...

I'm assuming he means the witness data part of the transaction, which is where ordinals are stored. The witness data is required to verify the state of the chain, but can be discarded ("pruned") after verification, to save storage space, usually. A broad repudiation of ordinals' claim to being "permanent storage" could be achieved if node runners ran modified core software that pruned ordinals witness data.