[mimixco]

It's only relevant in that Satoshi took the proof of work idea for Bitcoin from a spam prevention system like the OP discusses.

A lot of cryptos try to get around proof of work (proof of stake, for example) because PoW takes a long time and uses a lot of electricity, and this is the paper's main complaint -- but so far it's proved quite secure for Bitcoin. The Bitcoin blockchain has never been successfully hacked.