Fast “preview” snapshot path? (GPU previewImage availability)

I’m trying to use a GPU-backed preview snapshot from UXP to grab a downscaled composite of the current document

In some Beta builds I’ve seen reports/examples that suggest a DOM method like:
const pm = await app.activeDocument.previewImage({ maxDimension: 1920 });
const bytes = await pm.encode({ format: 'jpg', quality: 9 });

but I can’t find official documentation – does anyone know anything about this?