Snippet #009 Custom Gutenberg font sizes https://since1979.dev/wp-snippet-002-changing-the-gutenberg-font-sizes/

disable_custom_font_sizes.php

<?php
/**
 * disable_custom_font_sizes.
 *
 * Disable the custom font-size options in the Gutenberg editor
 *
 * @see https://since1979.dev/wp-snippet-002-changing-the-gutenberg-font-sizes/
 *
 * @uses add_theme_support() https://developer.wordpress.org/reference/functions/add_theme_support/
 */
function disable_custom_font_sizes()
{
    add_theme_support('disable-custom-font-sizes');
}

/**
 * Hook: after_setup_theme.
 *
 * @uses after_setup_theme https://developer.wordpress.org/reference/hooks/after_setup_theme/
 * @uses add_action() https://developer.wordpress.org/reference/functions/add_action/
 */
add_action('after_setup_theme', 'disable_custom_font_sizes');
?>