This was the true, final answer that solved for the exact scenario described in the original question:
/** * Disable Native Gutenberg Features */function gutenberg_removals(){ add_theme_support('disable-custom-font-sizes'); add_theme_support('editor-font-sizes', []);}add_action('after_setup_theme', 'gutenberg_removals');Passing an empty array to the editor-font-sizes avoid a PHP notice for an invalid argument.