Yes... well. We have it working, but not turned on for guests.
It introduces some issues though, which is why we've hesitated to turn it on for everyone.
1. If you have never had an account on MacRumors, it works fine (assuming it was on), though it uses a blank icon for the sign in page, instead of the MacRumors icon.
2. If you simply want to link your current account with Apple Sign in as an alternative method to login -- that works right now. Visit:
https://forums.macrumors.com/account/connected-accounts/ while logged into your current MacRumors account. Though you can't login with it yet, since it's not active on the Login page.
However...
3. If you have a MacRumors account and want to _replace_ the email address with an Apple Sign In one... then it's gets a little more complicated.
The solution depending on what your current email address is... would require creating a new account, and having us merge them. But that' not possible if your current MacRumors email address is the same as your Apple ID email address. In that case, if you try to create a "new" account, it will simply link your existing account. So maybe the solution would be to change the email address of your current account, and then create a new account to merge?
Anyway, that's what has held us up to opening it up.
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