The MyBB development team announced that they will be concluding maintenance and support for the MyBB 1.6 series at the 1st of September, 2015. MyBB 1.8 will be released for one year by that date and MyBB 1.6 will no longer get maintenance or security updates.
There is no LTS release for MyBB.
If you are still using MyBB 1.6 you should update your forum software, plugins and theme to MyBB 1.8 as soon as possible. If you need support with the plugin update check out this tutorial.
MyBB 1.8 comes with nice features to support your MyBB business too, so hurry!