The library has lots of security bugs that will not be fixed, arch linux has removed the fancy plugin from claws-mail. Is a fancy replacement scheduled at some point?
There is a similar movement at the Fedora Project:
Proposal: remove insecure WebKitGTK+ packages for F27
Gentoo removed the plugin too: https://bugs.gentoo.org/show_bug.cgi?id=608612
Arch Linux is now offering dillo as a replacement, which however in my opinion is far inferior.
FWIW, the corresponding Debian bug: https://bugs.debian.org/790199
As of version 17.10/19 October 2017, the fancy plugin is no longer available in Ubuntu, because of this issue.
This bug has has a non-negligible effect on my productivity. Although I'm not an experienced C/Gtk dev, purely out of idleness I started poking at this to see how much work was involved.
Incomplete code is at: https://github.com/khaeru/claws/pull/1