Gutenberg Snippet: Modify the first paragraph block added by default on pages with a custom placeholder text

custom_paragraph_block_placeholder_for_pages.php

function custom_paragraph_placeholder_for_pages() {
	
	//Define your post types to lock for classic editor block. Default CPTs are 'post', 'page'
    $post_type_object = get_post_type_object( 'page' );
    
	//Define what blocks you want to lock it for. Here is the list of core blocks:
	//https://gist.github.com/munirkamal/0d443605a3273926051c2971d5f3ff84#file-core-blocks-txt
	$post_type_object->template = array(
        array( 'core/paragraph', array(
            'placeholder' => 'my placeholder...',
        ) ),
    );
}
add_action( 'init', 'custom_paragraph_placeholder_for_pages' );