Now that we know the sample is the last track, and what Lana said to W Mag about “saying thank you and goodbye” on every closer of her records, I don’t think it’s Cinnamon Girl. I think a strong contender is The Greatest since that song feels very much like closure from everything.
Since Lana, Jack, and Mike are writers on the song, it has to be a Jack track from 2019–present (which rules out anything from Blue Banisters, and it’s probably not something from COTCC, since it’s fairly recent to have any “shocking” fan favourites). It’s either an NFR! track OR it could be something from 2006-2008 when she was writing mostly on her own.
That said, a sample usually means it’s related to an instrumental, because Lana has self-referenced before with lyrics, but that’s merely a reference - not a sample. So I really think it’s The Greatest.