Setting “Open in New Tab” for Link Control to be checked by default

You have to watch editor changes, you can hook the js to ‘enqueue_block_editor_assets’.

your-child-theme/js/default-open-in-new-tab.js

wp.domReady(() => {
    //console.log("Custom script loaded");

    const { subscribe } = wp.data; //we want to subscribe to changes of the editor

    // Helper function to determine if a URL is external
    const isExternalLink = (url) => {
        try {
            const domain = (new URL(url)).hostname;
            return domain !== window.location.hostname;
        } catch (e) {
            return false;
        }
    };

    const updateLinksToOpenInNewTab = () => {
        const blocks = wp.data.select('core/block-editor').getBlocks();

        blocks.forEach((block) => {
            if (block.attributes && block.attributes.content) {
                const parser = new DOMParser();
                const doc = parser.parseFromString(block.attributes.content, 'text/html');
                const links = doc.querySelectorAll('a');

                links.forEach((link) => {
                    const href = link.getAttribute('href');
                    if (href && isExternalLink(href)) {
                        link.setAttribute('target', '_blank');
                        link.setAttribute('rel', 'noreferrer noopener');
                    }
                });

                const updatedContent = doc.body.innerHTML;

                if (updatedContent !== block.attributes.content) {
                    wp.data.dispatch('core/block-editor').updateBlockAttributes(block.clientId, {
                        content: updatedContent
                    });
                }
            }
        });
    };

    subscribe(() => {
        updateLinksToOpenInNewTab(); // when editor's state changes, this happens
    });

   // console.log("Subscription added to update links to open in new tab.");
});

functions.php

function enqueue_custom_editor_script() {
    wp_enqueue_script(
        'default-open-in-new-tab',
        get_stylesheet_directory_uri() . '/js/default-open-in-new-tab.js',
        array( 'wp-hooks', 'wp-element', 'wp-dom-ready', 'wp-edit-post' ),
        filemtime( get_stylesheet_directory() . '/js/default-open-in-new-tab.js' ),
        true
    );
}
add_action( 'enqueue_block_editor_assets', 'enqueue_custom_editor_script' );

Got this together thanks to ChatGPT4. It would be probably a more native way to write it in JSX and have it compiled by Babel.