Since this is the sort of error that comes up a lot in Arch Linux, I've written a specialized script to analyze packages and determine if they are correctly built. (In 95% of cases the Arch Linux package is correctly built): https://github.com/eli-schwartz/dotfiles/blob/master/bin/pkg-list-linked-libraries
Running this on the bind-tools package and looking for libidn2 shows that the latest version of bind-tools is linked to libidn.so.4:
$ pkg-list-linked-libraries bind-tools libidn2
==> checking linked libraries for bind-tools-9.13.5-4-x86_64.pkg.tar.xz ...
Where does that library come from?
$ pacman -Qo /usr/lib/libidn2.so.4
/usr/lib/libidn2.so.4 is owned by libidn2 2.1.0-1
The bind-tools package in Arch requires the libidn2 package as a dependency, according to the output of
pacman -Si bind-tools, which is well and good since it depends on it.
The versions of the two packages in the official Arch Linux repositories at the time of this writing correspond to the version of bind-tools and libidn2 that I've checked above:
$ expac -S '%n %v' libidn2 bind-tools
So, you plainly have the latest version of bind-tools installed. But you claim you don't have the libidn2 libraries the binaries require. Why not? What is your version of libidn2 that is currently installed? Check the output of
pacman -Q libidn2.
Since the package works for thousands of other people, my conclusion is that your system is broken, and you need to properly update your system in order to install the latest version of libidn2. See https://wiki.archlinux.org/index.php/System_maintenance#Partial_upgrades_are_unsupported for details on why this might happen, what to do to properly fix it, and why symlinking is not only not the solution, but will break your system even further.
As for the Parabola bug, that is the exact opposite problem. A completely different Linux distribution that is not Arch Linux, failed to rebuild their packages using the new libidn2, and as a result those packages were erroneously linking to libidn2.so.0
If you're really suffering from that Parabola bug, then you're not using Arch Linux at all, and you need to specify that you're actually using Parabola, because otherwise people won't know how to help you.