Spark is an implementation of a statechain, which does something different than a covenant: statechains enable the offchain transfer of UTXOs by essentially passing around the private key to a UTXO offchain. The reason this is considered secure is because it's actually only one private key in a 2-of-2 multisig, with the other key owned by a central "statechain entity", who "promises" to only co-sign transfers that conform to the protocol. There are more details to it than that, but that is the gist. I'm not aware of any improvements that covenants can make specific to statechains.

A "real world" example of a key deletion covenant would be what Rewind wallet does.