Thanks to user brunnerh I dug deeper into it and found the root cause: <svelte:self> is also deprecated:

import Node from "./Node.svelte"
<Component {node}>
    {#if hasChildren(node)}
        {#each node.children as child}
            /* This is deprecated: <svelte:self node={child} /> */
            <Node node={child} />
        {/each}
    {/if}
</Component>

After updating <svelte:self /> by referencing the same component again, I have the correct export children() i needed to use {@render children()}:

<script lang="ts">
    import type { ListItem } from 'datocms-structured-text-utils';

    let { node, children }: { node: ListItem, children (): any;  } = $props();
</script>

<li>{@render children()}</li>

Thanks mates.