An unlimited number of Open Timestamps can be put onto the blockchain with just 32 bytes (plus some overhead) thanks to the magic of Merkle trees.  You then have to keep the path from your hash up to the root, and all siblings long that path, as proof that your hash was created prior to the block being mined.  There is a service that collects all these timestamps, builds the Merkle tree and pays to put the final result onto the blockchain. This should be supported somehow (though I'm not sure the actual expenses are) as it is becoming (or will soon become) critical infrastructure.

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