WordPress team retired and deprecated the 2.0.x legacy branch, 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’ve been a monumental task and could’ve introduced instability or new bugs. We had to make hard decisions between stability and merging in the latest security enhancements.”