Dust Expiry without Confiscation
Introduce a rule that makes very small, very old outputs prunable. However, retain the location of that UTXO—e.g., location = block_height/tx_index/output_index—indicating where in the blockchain the UTXO was created.
Users can still spend their UTXO if they include, in the annex of their spending transaction, an SPV proof (a Merkle inclusion proof of the transaction that created the UTXO, in a block in the chain).
This shifts the burden of storing the data from the global UTXO set to the user.
An expired UTXO’s location can be represented in less than 8 bytes. And since a set of locations has lower entropy, it is likely compressible to less than 8 bytes per expired UTXO.
This reduces spam by a factor of more than 8. In practice, that means of those +5 GB of spam, we could prune more than 4.3 GB. In particular, it would allow us to prune all of the bare multisig spam.