fcitx-sogoupinyin maintainer here. The package is NOT actually broken.

What yan12125 pointed out is just a taste of how to use old versions of a library: change the dependency to another package which contains the old version, or create symbol links from the new version. As symbol link just works and I don't want to replace the package libidn with an AUR package (the old version was only available in AUR at that time), I choosed the symbol link way. Now that libidn11 has been available in the community repository several hours ago, I have replaced the dependency libidn with libidn11.