If you hear of a judge that encourages juries to think for themselves and answer as they think, rather than prescribing how to think to them, let me know.
Jury nullification has never existed as a right in any common law system. It is merely a descriptive term that lawyers use to describe a situation where a jury decides to ignore the law. Lawyers cannot instruct juries on its existence because, in the law, it doesn't exist. It only ever happens because juries cannot be punished for deciding contrary to what the law says; their right to their own reasoning is sacrosanct.
It describes a situation where the jury ignores the law, and lawyers can never instruct juries to ignore the law, in any circumstance. Lawyers can never say "I know the law says X but if you ignore that, we can't punish you hint hint."
It is a term that describes a crinkle in the law. It does not exist as a right, and juries are NOT entitled to do it, but at the same time, they cannot be punished for doing so.
People who whine about jury nullification are one step below 'sovereign citizen' types in terms of driving me nuts with their willful misunderstanding of the law.
As the other posters said, most jurisdictions follow specific recommendations on avoiding telling the jury to do whatever they want. And different jurisdictions have different rules on whether or not the jury can truly redefine the law or facts. Some places have that ingrained in their state constitution, but most do not. And civil vs. criminals trials also have different rules. And where I live, there are even different rules for specific types of civil trials.
It really is all fairly complex, hence every trial will give a specific set of instructions to that jury. And that is also why an attorney in one jurisdiction might not be correct on how the details work in another one. (Although the gist of it all will be the same.)
At the same time, this is also why we don't know for sure what the jury said or thought. We can get all their instructions - those are public record. But the actual deliberation is not.
As always, IANAL. Other commenters already seem to be annoyed at my layman's take on it all, but this is what I know from being a law school dropout and having served on juries.