The proposal has been implemented as a Swift package, with a PR opened on the Swift Evolution Staging repo: AdjacentPairs by mpangburn · Pull Request #2 · apple/swift-evolution-staging · GitHub
You can read the latest version of the proposal via the original Swift Evolution PR: Add an `adjacentPairs` algorithm to Sequence by mpangburn · Pull Request #901 · apple/swift-evolution · GitHub
Thanks @nnnnnnnn for your guidance!