I’m afraid the general answer is still largely the same as last time
The spec I was talking about is OpenID Connect Session Management. Unfortunately, that iframe-based solution is becoming increasingly less useful because many browsers are now blocking third-party cookies by default. It now only works reliably if your identity provider and Discourse have the same ‘origin’.
As @simonk said, depending on your identity provider it might be possible to implement something custom via a theme component, but I’m not aware of any general solution we could add to Discourse itself.