Caveat
From the REST API docs:
Changing or removing data from core REST API endpoint responses can break plugins or WordPress core behavior, and should be avoided wherever possible.
That said, there’s a rest_post_query filter (well, rest_{$this->post_type}_query) that will let you modify the WP_Query that’s grabbing your posts, so you should be able to add a meta_query to the query.
eg:
add_filter( 'rest_post_query', 'wpse405394_remove_global_posts', 10, 2 );
/**
* Removes the "global_post" posts from the REST API.
*
* @param array $args The WP_Query arguments.
* @param WP_REST_Request $request The REST request.
* @return array The filtered $args.
*/
function wpse405394_remove_global_posts( $args, $request ) {
if ( empty( $args['meta_query'] ) ) {
$args['meta_query'] = array();
}
$args['meta_query'][] = array(
'key' => 'global_post',
'value' => true,
'compare' => '!=',
);
return $args;
}
This code is not tested, and may break things in your site. A lot of things use the REST API under the hood (eg: the block editor). Test thoroughly.