Here's a bit of inside baseball when it comes to running a service like Buttondown: there's a tricky balance to achieve between maximizing author outcomes and respecting subscribers. On one end of the spectrum, we could be really annoying and send a ton of emails to subscribers, but that would be bad for authors. On the other end of the spectrum, we could be really respectful and send no emails to subscribers at all, but that would be bad for authors.
For a while, authors have been able to manually send a reminder to subscribers who haven't confirmed their subscription. This is a great way to get a few more subscribers on board, but it's a bit of a pain to do manually.
So... we automated it. In your settings page (under "Subscription flow"), you can now enable automatic reminders for subscribers who haven't confirmed their subscription. We'll send them exactly one email: roughly 24 hours after they initially signed up (with a bit of juice to make sure it's sent at a reasonable time of day for them).
I've enabled this for all current authors, but it's a one-click toggle to turn it off if you don't want it.