functions.php

<?php
/**
 * Enable email-based 2FA by default.
 *
 * @author pjv
 * @link https://link.from.pw/3P7cH9u
 *
 * @param $providers
 * @param $user_id
 * @return mixed
 */
function pw_default_2fa( $providers, $user_id ) {
	$force_roles = array(
		'administrator',
		'editor',
	);

	$user = get_user_by( 'id', $user_id );

	if ( empty( array_intersect( $force_roles, $user->roles ) ) ) {
		return $providers;
	}

	if ( empty( $providers ) && class_exists( 'Two_Factor_Email' ) ) {
		$providers[] = 'Two_Factor_Email';
	}

	return $providers;
}
add_filter( 'two_factor_enabled_providers_for_user', 'pw_default_2fa', 10, 2 );