Hi @mhautala
Thanks for contacting Auth0 Community.
As per my last post on Account Linking via Actions here https://community.auth0.com/t/linking-passwordless-email-identity-to-primary-identity/97535 this is still unsupported, however, this is being worked on as we speak so support is coming very soon indeed. I expect an announcement will be made here Auth0 Changelog when it is ready.
My understanding is that account linking via Rules has a known vulnerability in it, this is why it was removed from our Rule samples. At this moment in time, I would suggest either waiting for the Actions release or implementing your other workaround depending on urgency of your requirements.
Warm regards.