The Code of Conduct did not specifically state a preference for using pronouns when a user's pronouns are unknown. I personally use singular they for this purpose. Others might use a different pronoun, and that's ok, when the user's pronouns are not known. In that sense, there is no "preferred pronoun".
However, when the user's pronouns are known (particularly if they are expressly stated by the user), everyone is expected to make a good faith effort to use them. This isn't a "preferred" pronoun either. It's the person's pronoun.
Generally speaking, using she or he is easy to understand. Most people are familiar with those pronouns and are polite about being corrected. For instance, we can understand not wanting to be called he when we're actually she, even if we don't really mind if that happens to us specifically. So that part of the policy -- let's make sure we respect people's pronouns -- makes sense to pretty much everybody. If you know I'm he and you consistently refer to me as she, even after I have asked you not to, that violates our fundamental "Be Nice" policy.
Where it gets harder is when we talk about gender identity. Online, we're just usernames. I can tell you I'm he without any baggage of your perception of my gender, and we're not sharing a bathroom, so I'm not in a position where I have to justify my identity. I can just be he because I said that's who I am.
But not everyone feels like they fit neatly into a binary gender pattern, meaning they don't necessarily feel like he or she fits right. They might feel gender-neutral, or gender fluid (their experience changes over time), or somewhere sort of in-between he and she, and we call this "nonbinary gender" (or enby). I can't describe the enby experience because I'm not, but all I really need to know is that they feel like a different pronoun is the right pronoun for them. That's commonly they -- it's by far the most common amongst all the enby people I know -- in part, I think, because we are already used to using singular they when gender is unspecified or to specifically obscure gender. There are also neopronouns, which Tonepoet has linked resources for.
I believe these usages are in the OED, but even if they are not, I encourage you to use them when asked. Here is the thing. It's not about whether a person's pronoun is grammatical in your perception of what's proper. It's about that person's identity as a human being. I would be hurt and upset if other people argued whether it was grammatical to refer to me as he, and if they were further offended when they offered to refer to me by some other pronoun that was "proper" so long as it was not that one and I said no because I'm he not some other pronoun.
Again, the change to the Code of Conduct is not intended to punish people who are trying. We know it's difficult, especially for non-native speakers and especially for non-native speakers of gendered languages. The important thing is that we all try to respect people's identities.
Thank you for asking. I know that many, many, probably all of my transgender and genderqueer friends are tired of having to explain it to everyone. I have done my best to accurately represent their viewpoint in an effort to spare them having to do it again, but I welcome education if I have missed the mark.