What is the meaning of the "spent" attribute of a transaction output?
Or is that attribute added &quot;later&quot; by blockchain.info to give the reader a bit more context?

I'm pretty sure that is what is happening.
If you look at that transaction in a different explorer, for example https://blockchair.com/bitcoin/transaction/c52b5df64f10463fbe4f46d7b9da3464449849b7e542d533575a927366b52b09 and click &quot;raw tx&quot; you'll see it doesn't include that information in the JSON.