In Quarto 1.3, callouts are represented as a custom AST node. You can create callout AST nodes in Lua filters with the quarto.Callout constructor. The constructor takes a single parameter, a table with entries type, title, and content, as described below. In Lua filters, callouts are represented as a table with the following fields:

type: the type of callout: note, caution, warning, etc (optional in the constructor).
title: The callout title (if any) (optional in the constructor),
icon: the callout icon (or false if none) (optional in the constructor)
appearance: "minimal", "simple", or "default" (optional in the constructor)
collapse: whether to render the callout as collapsible (optional in the constructor, default false)
content: the content of the callout (a pandoc.Blocks object, or a plain list in the constructor)