It's definitely Let's Encrypt with Certbot because I went and had a look through my emails when I saw this thread. But I can't even remember installing it all. And ACME does ring a bell. I suppose I should go and check before it all stops working. I do vaguely remember that there's a limit of around a couple of years before I have to reapply to use it - presumably as opposed to expiry.
Anyway, my more immediate concern is whether to update to the latest version of OwnCloud since they withdrew their nice simple upgrade script.