"The AO3 database contains account information that may include your username, email address, IP address, and a hashed version of your password."
Dear lord. Can you imagine the damage that could happen just by leaking everyone's username, email address, and all the stories they've written/kudos'ed/bookmarked? Like a miniature version of the Ashley Madison hack.
Best practice to use your non-primary email address for these kinds of site. Data breach is a matter of when, not if, it happens.
Did anyone sign up to that with their real name or email?
It's worth noting that often, the email can be correlated with other breaches to get your real name. People can do that relatively easily on osint websites. I wasn't terribly pleased when I found my teenage furaffinity account show up in an identity theft monitoring app. Other people have turned up in breaches of far more objectionable websites.
If your email provider offers email aliases, you should really get familiar with how to use them, and use a unique email alias per website.