Intercept and prevent Shield's 2FA email sending process

functions.php

<?php declare( strict_types=1 );

/**
 * Perform any secondary checks before Shield verifies the 2FA nonce.
 * If any of your checks fail, throw a new \Exception() with the error message you'd like to display to user.
 */
add_action( 'shield/2fa/email/pre_send_email/pre_nonce_verify', function ( \WP_User $user, string $plainNonce ) {
	// e.g. this is a trivial example and not an actual check that is required.
	// The message 'User ID is invalid.' will be displayed to the user.
	if ( $user->ID < 1 ) {
		throw new \Exception( 'User ID is invalid.' );
	}
	if ( \strlen( $plainNonce ) > 100 ) {
		throw new \Exception( 'Nonce value is too long.' );
	}
}, 10, 2 );

/**
 * Alternatively, you can perform any further checks after Shield has verified the 2FA nonce.
 */
add_action( 'shield/2fa/email/pre_send_email/nonce_verified', function ( \WP_User $user, string $plainNonce ) {
	// e.g. this is a trivial example and not an actual check that is required.
	// The message 'User ID is invalid.' will be displayed to the user.
	if ( $user->ID < 1 ) {
		throw new \Exception( 'User ID is invalid.' );
	}
	if ( \strlen( $plainNonce ) > 100 ) {
		throw new \Exception( 'Nonce value is too long.' );
	}
}, 10, 2 );