What I have in mind are proof of existence software, than are a kind a non-centralized and virtual notary service. They time-stamp digital information submitting a hash to the blockchain.

I know already that technically, the blockchain can be used for that. After all, it's basically a time-stamping system.

However, is this kind of service disruptive to the bitcoin community? Can it become disruptive when it reaches a certain level and be curtailed?

I also wonder whether the messages are prunable. Important are the transactions, not the associated messages. If the blockchain grow and grow, would the messages be still be kept?

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There are already tons of services doing this, so probably worth focusing your creative efforts elsewhere. Here are the ones we know about on Coin Secrets:

As for pruning, assuming you use the same OP_RETURN mechanism as these tools, then they may be pruned by some nodes. But I imagine there will always be plenty of full archives of the blockchain available (even if this is eventually a paid-for service) so I doubt that your timestamped hashes will ever be truly lost.

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