INN is still actively maintained, and has perfectly fine IPv6 support built right into it.
C-News is basically dead, and you don't really want to use it on a busy news server - but for the fun of it, I decided to port the NNTP reference implementation to use IPv6 transport if available. It's running since 2002 on a NetBSD/Sparc system on a SPARCstation-LX with 50 MHz and 64 Mb RAM - but it does the job :-)
The patch is a bit more than just IPv6 support - a few library functions had to be renamed due to collisions with system functions, and a few definitions had to be commented out to make it compile.
Here is the patch, and here is the full source (159 kbyte) of the NNTP 18.104.22.168 reference implementation.
-rw-r--r-- 1 gert other 12170 Dec 30 11:51 nntp-22.214.171.124-IPv6.patch -rw-r--r-- 1 gert other 159146 Feb 23 2002 nntp-126.96.36.199-IPv6.tar.gz(A basic C-News installation does not have NNTP support at all - so you first need to install C-News from the original sources, and then add NNTP 1.5.* to it)