The WordPress team had initially committed to maintaining the WordPress 2.0.x legacy branch until 2010. Unfortunately, we bit off more than we could chew—the 2.0.x branch is now retired and deprecated, a few months shy of 2010.
Many of the security improvements to the new versions of WordPress in the last couple of years were complete reworks of how various systems were handled. Porting those changes to the 2.0.x branch would have been a monumental task and could have introduced instability or new bugs. We had to make hard decisions between stability and merging in the latest security enhancements. Additionally, far fewer people stayed on the 2.0.x branch than we anticipated. I take that as a testament to the new features in WordPress and perhaps even more the features offered by plugins, many of which don’t support older versions of WordPress!
I’m disappointed that we weren’t able to keep the branch maintained until 2010, but since one of the big reasons for that failure was the massive scope of our security improvements for the newer versions of WordPress, 2.0.x doesn’t die in vain!