Log in to any forum with the same account and email privacy (for hosted-plans)

That's because you're admin. (Maybe you knew already, just clarifying, in case someone else doesn't know.) Only admins can see others' email addresses.

Makes me think: 1) Maybe it'd be good to have to click an extra time, to see someone's email address. And 2) some time later, there might be a config option specifying if moderators should be able to see people's email addresses. Maybe there'll be 2 levels of moderators: "basic" mods who cannot see email addresses, and "full" mods, who can.

It would be nice if the hosting-plans allowed users to log-in with the same account on all forums, and the email database was kept in a way that forum owners would not know the user's email.

Yes. I've been thinking a tiny bit about global identities, and then later about how Talkyard maybe could integrate with Matrix. And the Matrix people have devised some kind of identity server and API; maybe Ty could use this, and in that way people could log in to Talkyard forums, with their Matrix identities (if a forum allows this, and the person wants to).

They (the Matrix people) are also thinking about decentralized user accounts: https://github.com/matrix-org/GSoC/blob/master/IDEAS.md#decentralised-accounts
where an identity would in essence be a public-private key pair: https://github.com/matrix-org/matrix-spec/issues/246.

Maybe by "just waiting" until Matrix has decentralized user accounts, Ty could get both software for an email database & just-a-username login, so the admins didn't need to know people's emails, and some Matrix interoperability as well :- )

(Then this would work also for self hosted servers, if those admins wanted to.)