The Yii forum is based on IP.Board 3.2.x.
IP.Board 3.4.0 has been released and IP.Board 3.2.x has reached EOL: http://community.invisionpower.com/topic/373381-ipboard-340-and-ipnexus-155-released/
Particularly…
Security updates will still be provided for "several months", although "several months" is not an exact date and I think that a board upgrade should be planned within some months.
As I was informed once, by one of the forum moderators (probably mdomba), Yii forum is extended, in a large scale manner, from base IPB source files, to support integration with other framework sites (like docs, extensions etc.) and to provide a lot of additional functionallity.
Updating IPB to newer version is not as easy as installing new version of board engine. It would rather require to rewrite a lot of customized code from scratch. Since all this effort and all, really large modifications, were done in moderators’ spare time, I don’t think this is possible.
There are a few, smaller or bigger bugs already reported, but I think, in this situation, we’ll just have to learn to live-on with them.
Until the next defacement? 
We plan updating to latest version but as it was mentioned, it’s not easiest thing to do because of custom additions.
You could wait for IPB 4.0 which is under development.
Updates are incremental anyway so there’s no big difference.
Yeah but it’s a major upgrade so if you have some integrations/bridges maybe they could need major changes.