In the 2.x docs, all versionadded strings should stay. But IMO in the 3.0 docs we should get rid of them all. If you want compatibility information, look at the 2.6 docs (those should also mention things that are changing in 3.0).
I agree. People who target 3.x need to test anyway if they also want to support some 2.x version (if that is possible at all), so it does not help them to know what Python version introduced a certain feature they use.