Given that we're now talking about VST3 and VST2 and (presumably) VST1, and that the world is full of persons who wrote plug-ins that didn't really write to the the spec all along, I'm not sure that your question - as phrased - is one that any mindful person would be able to confidently reply to. Perhaps a more specific question would yield you a better answer.

Tell me about it! As the user base for our own audio plugin host has expanded, we’ve had to do some bizarre defensive programming to protect ourselves from some of the more egregious mistakes made by some plugin developers, even some from well-known vendors.