I have been advised to use the following template, which I have just now sent. I'll report back with what happens...
email: privacy@substackinc.com (and cc tos@substackinc.com)
"I am an Australian resident requesting the immediate deletion of my account and all associated data under the Australian Privacy Principles (APP 12).
Substack has made it impossible for me to delete my account via the standard interface without submitting to invasive biometric verification, which I refuse to do.
I am instructing you to terminate my account associated with this email address immediately. If you require verification, verify this request by replying to this email address, which proves my ownership of the account. If you refuse to delete my data without government ID, I will lodge a formal complaint with the OAIC (Office of the Australian Information Commissioner) for holding my data hostage."
I tried this, and got a reply with a link, that redirected me to the age verification again. I replied to this asking them to manually delete my account, and they manually verified that I was an adult without asking for bio-metric verification. I just my calm, but critical emails convinced them, along with my paid subscription history that I was an adult :).
Oh so you can log in and use the site as normal again now?
I emailed privacy@substackinc.com back and forth a couple of times and today they deleted my account and refunded my last payment. They were very polite and helpful.
email: privacy@substackinc.com (and cc tos@substackinc.com)
"I am an Australian resident requesting the immediate deletion of my account and all associated data under the Australian Privacy Principles (APP 12). Substack has made it impossible for me to delete my account via the standard interface without submitting to invasive biometric verification, which I refuse to do. I am instructing you to terminate my account associated with this email address immediately. If you require verification, verify this request by replying to this email address, which proves my ownership of the account. If you refuse to delete my data without government ID, I will lodge a formal complaint with the OAIC (Office of the Australian Information Commissioner) for holding my data hostage."