From gnupg.org: "2.0.26 is the stable version suggested for most users, 2.1.1 is the brand-new modern version with support for ECC and many other new features, and 1.4.18 is the classic portable version."
The 2.1 series of gnupg is not stable, it still has many major bugs, not the least of which is backwards compatibility with various key sizes previously supported (introduced by the new gpg to gpg-agent IPC layer restrictions). On the gnupg-devel mailing list I've seen a few potentially serious security issues with it. Given that it's not marked as stable upstream, and that it's such a critical core component of Arch's infrastructure, I find it questionable for Arch to have upgraded so soon. In the future, can we avoid upgrading gnupg proper to non-stable releases? Instead, how about creating a gpg21 or gpg-modern package would be appropriate for those wishing to try the unstable version and then updating the gnupg package once that branch gets marked stable?