That is incorrect, there is no such mentality and has never been any such mentality. Of course no new features is added to 3.0, because it's unsupported nowadays, Python 3 is up to 3.2, and 3.3 is coming soon.
There was a moratorium for Python 3.2 where no more features was added to the language, to get other implementations than CPython a chance to catch up. But that was not a question of porting, and that did not include new features in general, just to the language. New features in the standard library and other implementation improvements was done in 3.2, and in 3.3 new language features are added again.
So the claim of a mentality of "avoid adding new features" is nothing but FUD and complete hogwash.
There was a moratorium for Python 3.2 where no more features was added to the language, to get other implementations than CPython a chance to catch up. But that was not a question of porting, and that did not include new features in general, just to the language. New features in the standard library and other implementation improvements was done in 3.2, and in 3.3 new language features are added again.
So the claim of a mentality of "avoid adding new features" is nothing but FUD and complete hogwash.