Disable Full Site Editing

<?php
// Remove the Site Editor entry from the Appearance menu.
add_action( 'admin_menu', 'scseed_remove_site_editor_menu', 99 );
if ( ! function_exists( 'scseed_remove_site_editor_menu' ) ) {
    function scseed_remove_site_editor_menu() {
        remove_submenu_page( 'themes.php', 'site-editor.php' );
    }
}

// Disable template-editing mode inside the block editor.
add_filter( 'block_editor_settings_all', 'scseed_disable_fse_template_mode' );
if ( ! function_exists( 'scseed_disable_fse_template_mode' ) ) {
    function scseed_disable_fse_template_mode( $settings ) {
        $settings['supportsTemplateMode'] = false;
        return $settings;
    }
}