So, here I am, because I did the same problem. I got the following response:
Greetings! Thank you for contacting The Elder Scrolls Online's Customer Support! My name is Kathy and I'll be assisting you with your unlink request.
I understand you are seeking an unlink from your Steam account that you have tied to your ESO account.
Due to a technical issue, we are currently not able to unlink accounts at this time. However, once the feature becomes available, we will see what we can do. There is no guarantee that we will be able to unlink, however, once the feature is there, we can review your case again.
In the meanwhile, we do require some information from you to help process your request later.
Meanwhile, if you had previously been playing using our ESO Launcher and you are wanting to play your characters that you already had before linking, log in using the launcher from the ESO website and not the Steam launcher and you will have access to your characters. You will also want to do any purchases of Crowns or ESO Plus through our website and not the Steam website.
If you have already provided this information to me directly, please disregard the above message as we have documented your information.
If you have any additional questions or concerns, please let us know and we'll be happy to help. Replying to this email will re-open the ticket, if it closes, and we will be back with you as soon as possible.
Regards, Kathy The Elder Scrolls Online Team