Allow underscore character in domain host name

Because underscores are not allowed in the domain name specification.

Many resources on the subject, e.g. https://docs.aws.amazon.com/Route53/latest/DeveloperGuide/DomainNameForm...

You can use characters other than a-z, 0-9, and - (hyphen) only if the top-level domain (TLD) supports IDNs and supports the language that you want to use.

If you are using i18n domains, that is a different issue.