@pndc It's very likely that you actually signed up for (free) monitoring some years ago and simply forgot about it.

I also have a single-user domain with a catchall address and signed up for free monitoring, and yes, I did bump up over the threshold into paid monitoring at some point. But I'm still the one who asked to be notified. Pretty sure that's what happened for you!

If you only have one real address at that domain, just subscribe for updates for that one address, and unsubscribe from the domain-wide ones. Then you should never see that email again.