There are some timestamping services such as proofofxistence.com that use OP_RETURNS transactions to store a document's hash, so in the future a 3rd party can verify the existence of the hash (and the document) at that particular time.
One of the assumptions of using the blockchain for trusted timestamping is that the data will be there for a long period of time. However, the OP_RETURN ouputs are unspendable (then pruneable). It can be theoretically forgotten by everybody without affecting the integrity of the blockchain.
Is it a good idea to use OP_RETURN transactions for long-term timestamping?