I notice that when logging into this forum a number of SSO type options are presented e.g. Sign in with Apple, sign in with LinkedIn etc. (but not with Google 
) . How secure is it in general (not just to this site) to use such methods of login ? Does is not contradict the policy of having different logins for different sites ?
Also does it raise privacy questions ?
Being able to log in with your passkey directly is quite convenient and secure.
Options are only given to you for you to decide how you want to authenticate your account when opening one and using it. You have complete control over that decision.
Also, login in general in has little to privacy and more/a lot to do with security and authentication. So what do you mean by contradiction you’re thinking of?
I’m thinking for example if your LinkedIn account was compromised and you are using LinkedIn as your login for privacy guides or other website does that not render your privacy guides as compromised also as the LinkedIn attacker can access your privacy guides account also. In other words compromising the Login as account renders other accounts vulnerable as well - much as using the same password on multiple websites does ?
The operative word there is “if” - if you have good opsec, that’s a big if and one needn’t worry.
Also, that’s why 2FA and passkeys exist. If one account is compromised that can also compromise another account, without 2FA/passkeys, the attackers can’t do anything.
Ergo, it is always recommended to use a password manager with an alias as your username email with 2FA and/or passkeys enabled for every account you can.
If you don’t have use 2FA and a strong password, …