Sometimes I just shouldn’t be let near technology.
In short: I managed to accidentally delete the mailing list.
You know how you get the little checkbox at the top of a page to select all? Well, SendInBlue has that too. Except they don’t tell you that you’re selecting everything on that page… and everything on every page. I went to delete a page of junk Yahoo email addresses. Selected all on that page, and clicked delete. BOOM! Full mailing list gone.