All the commercial ones I've bought in the past year or so do, and ever since I think JUCE 7 there have been good libraries for open source projects that want to add the format.
I think there's still a lot of bad feeling about the fact that there are many VST2 plugins that are open source but nonetheless illegal (or at least tortious) to build.
Hopefully this provides a path for those VST2 plugins.
No. VST2 has nothing in common with VST3 despite similar name.
Why? In Juce isn't that a matter of choosing multiple build target?
If you have the source code and use JUCE then yes, you can convert the plugin to VST3. But if you don't use a framework then you need to port the code manually.
Notionally yes but the idea of keeping the DSP and the container logic separate is relatively new and lots of projects didn't do it.
(Actually the idea is very old and how most LADSPA plugins were made but some time in the mid oughts everybody forgot about it.)
If you're using JUCE and not using any of the VST2 features removed in VST3.