So, possible world semantics applies to propositions. We evaluate a proposition at a collection of worlds with some relation on them that captures the sense of possibility in question. A requirement of this is that the thing we evaluate must be either true or false in each possible world. It doesn't help analyse whether we can say a moral rule is truth apt while treating a moral rule as a fundamental constituent of language use. So, if we start the inquiry into the truth aptness of a moral rule with respect to a possible world semantics, we will have begged the question about their truth aptness to begin with.

If Witty is suspicious about saying 'The Paris meter stick is 1 meter long' and 'The Paris meter stick is not one meter long', one wonders why something even messier, like 'One ought not introduce security backdoors into communications software', would behave differently. Paradigmatic examples of good conduct may be unable to be said to be good for precisely the same reason as the Paris meter stick cannot be said to be 1 meter long.

Can something be good in one world but not in another? — Banno

Yes. It certainly seems so to me. Why couldn't it?

Are the things that are good in all possible worlds the things on which we ought base our ethics? — Banno

First, can you give an example and demonstrate how it counts? If not, then I rightly dismiss your question.

So, if we are going to talk about necessary goods, we need first to affirm that goods are true, and then sus out which goods are true in all possible worlds... — Banno

When you catch that wild goose, would you be so kind as to let me know?

“So can a person have private morals?”

There can be no other way. Everything else is making others give you what you want by making them think it’s for their own good.

What you worry about when you’re lying in bed alone at night when everyone else is asleep, are your only real principles. The rest is just politeness, and if done in excess, showiness.