Taproot Assets uses SPV proofs in the proof file format for an asset. Each
asset starts at a gensis point, then proves that the rules of the overlay layer
were followed each each new state transition. Each state transition includes an
SPV proof of the transaction that commits to the state transition.

An upcoming revamp of the LN gossip protocol also has a cut out to optionally
include an SPV proof with a channel announcement message. This makes the
protocol more light client friendly, as otherwise a light client would need to
fetch tens of thousands of blocks to verify that a channel actually exists.

It isn't specified today, but a future extension to the `channel_update`
message could include a merkle proof of a _spent_ funding output to provide a
verifiable gossip layer proof that a channel has been closed. Today setting a
disabled bit in the channel update is used in place, but that's overloaded with
other signals (eg: the peer is offline).

> does not spend a coin

If the Merkle Tree were actually a Sparse Merkle Tree, then we'd have a
succinct way proving that a given transaction wasn't in a block. If such a
commitment were added over the set of spent outputs, then you could also
succinctly prove that a transaction wasn't spent in a block.

Such an STXO commitment would also be useful for SwiftSync as then peers are
able to verify the integrity of the undo data.


-- Laolu