As I announced before in this post, the MyBB team decided to drop the support for MyBB 1.6.
Since MyBB 1.8 has been released more than one year ago, this is a very valid decision. If you are still using MyBB 1.6 you should really upgrade to MyBB 1.8 now because you’d risk security issues otherwise.
If there are plugins that have not yet been updated to MyBB 1.8 compatibility they’ll probably never will and you should drop them.