Any idea what this is referring to?
schemes that make malleability irrelevant are subject to dangerous signature replay attacks if not handled very carefully
Is he saying that implementing
BIP 62 opens up a new known attack vector?
What I meant was the idea that what goes into transaction should be "open to the user".
Imagine you had a database and added to the ability to store arbitrary information into each row, this is why rational databases exist which require you to define the type of data you want to store before you do add that information. The game of whack-a-mole is because even when they remove malleability for necessary transaction data it still doesn't prevent that attack because each entry has "scrap space" after that.
My suggestion is to abandon that concept because it's not a sane approach to storing data but a software engineering nightmare.