Isn't that similar to the idea behind hashcash [1]? I don't know -- was hashcash used anywhere? Or were the ideas there leveraged in stuff like DKIM/SPF?
Hashcash is a different idea with the same goal of making certain email behaviors financially infeasible by tying emails to a more limited resource. The limited resource in IM2000 is -- well, that's complicated, I would say internet domains but someone else might say something like network availability. But in Hashcash it is clearly processor cycles.
Hashcash is "used anywhere" in the sense that it's the idea behind bitcoin. There's a duality here where the very introduction of limited scalable resources which makes a cryptocurrency possible, also can be used in a different way to make spam impossible.
In that duality it is actually kind of interesting to think about IM2000. One would imagine a cryptocurrency based on something like "proof of network bandwidth shared" or something, which would be really hard to theoretically formalize. But if you could get a secure definition then that fundamental idea becomes rather explosive. Like I imagine a sort of viral peer-to-peer filesharing network kind of like BitTorrent which would end up as a sort of alternative to the World Wide Web; whereas there are huge clusters of bitcoin miners right now trying to chug out more proofs-of-work, in that situation you would have large numbers of proxy hosts trying to mirror more and more files online.
Right now it would be possible to do some really nasty things to bitcoin by designing software which stores arbitrary files in the spare bits in the ledger. If that software becomes really widespread then inevitably someone uses it to upload MP3s or, worse, illegal pornography and those things get ossified into the Bitcoin ledger and you cannot remove public access to that content without taking down the entire blockchain; probably what happens in practice is that the sharing software itself gets demonized as "only pirates/perverts use that sharing software." But one is immediately confronted with concerns about "hey if I download the blockchain am I technically performing an illegal action" to which the legal answer is probably "yes" at that point. The law doesn't usually care about whether you need sophisticated software to decode that crap.
If you had a cryptocurrency that was based on "I hosted and transmitted data, but I don't know what that data was" then I think you would have a sort of robustness to the network, maybe, where the offending data is not in the ledger. With that said, probably it gets a similar stigma as "only pirates/perverts use that, all the rest of us use the web."
[1] https://en.wikipedia.org/wiki/Hashcash