This only affects signup forms hosted by MailChimp. If you build your own form and use their API for signups, there has never been a requirement for double opt-in.
So I don't know if this is such a dramatic change -- depends on how many people use the default forms instead of an integrated signup experience.
So I don't know if this is such a dramatic change -- depends on how many people use the default forms instead of an integrated signup experience.