Using the Juce Demo Plugin built from the latest develop tip of JUCE, if a session is saved with the VST3 format, the settings aren’t recalled if the same session is then loaded with the VST2. The same applies to presets.
Now, I’m not sure whether this should be possible, but it would be useful for collaboration between users who prefer to continue to use older machines (running DAWs that only support VST2, for instance) and those who use newer systems preferring to stay up-to-date.
It would be preferable not to force the users of newer systems to use VST2 when VST3 is available just so that they can still share sessions with the users who remain using VST2.