Can an ethical proposition be objectively true, empirically? I am persuaded no.
An ethical proposition is a statement of meaning, and a meaning without objective reference. Our meanings arise mentally. There are those Intuitionists who say there is an inner sense of right and wrong. But there are indications that this stand is very fragile. Human beings frequently show themselves very manipulable by group thinking and pressures, most obvious in such things as civilized Germany's descent into barbarism under totalitarian pressure and propaganda. Important experiments like the Stanford experiment, https://en.wikipedia.org/wiki/Stanford_prison_experiment
have shown further evidence of this human trait. Very quickly, under group pressure and persuasion, so-called deep sourced ethical meanings are dislodged. There are some brave and heroic (cf: Dietrich Boenhoffer) that stand with great decision and character against and hold out even to death But it is not the rule.
The ideas of Emotivism would seem to fit here also:
"Motivism is a meta-ethical view that claims that ethical sentences do not express propositions but emotional attitudes. Hence, it is colloquially known as the hurrah/boo theory." https://en.wikipedia.org/wiki/Emotivism
Stoic philosophy is built on the belief we can by choice and practice choose our ethical actions, and finally I am persuaded that this is what happens. There is no "moral reference stream" out there or in us but only our rational decisive faculty. An ethical position is taken in much the same way as we choose this car or that to drive around in. Of course, consequentially, the type and state of the car will determine our ride and those that ride with us. To stop everyone riding around in lemons, and ploughing into everyone else, we need laws which are a communal social choice by way of government. Consequentialism fits nicely with this understanding of ethical decision making.
"Before the sacred, people lost all sense of power and all confidence; they occupy a powerless and humble attitude toward it. And yet no thing is sacred of itself, but by declaring it sacred, by my declaration, my judgment, my bending the knee; in short, by my - conscience."
- Max Stirner
For me the 'conscience' of Mr Stirner is an inculcated social conditioning or alternatively our own, owned and developed one.