In T326937#9452283, @Tgr wrote:

@Tchanders should Special:CentralAuth/<username> be any different for temp users? E.g. show when the account will expire?

In T326937#9453870, @Tchanders wrote:

@Tgr We hadn't made any specific plans about showing anything different on Special:CentralAuth. But if we can reliably calculate expiry time (and also whether an account has expired) I can see that'd be helpful. FAO @Madalina too.

In T326937#9454937, @Tgr wrote:

In theory the expiry time is registration date plus $wgAutoCreateTempUser['expiryAfterDays']. In practice expiry is done with a maintenance script so the exact timing is unpredictable, but I don't think a small inaccuracy matters. We use the same date logic to show the temp users themselves a warning.

I don't think there's a good way to accurately tell whether an account is expired right now, other than guessing from the date. We expire them by invalidating the session; from the system's point of view the account is in a normal state, the user just doesn't have any session cookie to access it anymore. So I wouldn't show something like "this account has expired" because for that there's more expectation to be 100% correct. But "expires on <day>" seems OK to me.