I was thinking of something else with the OP_RETURN thing. it's unspendable which is the reason why it is safe to prune... there are so many ways to store arbitrary data on bitcoin it's hard to remember them all.

please read the STAMPS documentation

I also remember looking through an imageboard on zeronet where someone had made custom java code to embed arbitrary data into addresses on outputs. it looked completely novel like it was someone's hobby project. they were bragging about what they had done and they believed it would advance the conversation on legalizing CP.

someone suggested to fork bitcoin so that a person has to initialize an address before it can receive funds, which I guess would make this a little more difficult but not impossible. but that would be a ton of work, almost like starting over.