[0x0]

What's the stuff about "proof layers", is that new in this v2? The paper briefly talks about proof layer requests. Is this something merkle-tree related? What is the purpose? Is it to prevent clients from lying about having pieces they do not have by requesting a verifiable random hash chunk?

[the8472]

It's part of switching to merkle trees instead of flat piece lists. A merkle tree can only be verified if you either have a whole layer or send ancestor-siblings (uncle, great-uncle, etc.) along with a partial layer.

Merkle trees allow torrents to start faster from magnet links since only the tree roots need to be front-loaded while the tree can be fetched incrementally.