It appears that there are some hacking/cloning/phishing attempts involving substack.com. I recently received an odd email addressed to me and supposedly from me regarding notifications and a paid subscription, also.
Please note that I will never automatically upgrade your subscription to a paid subscription unless you initiate the upgrade.
Be on the look out. Go directly to the brianboley.substack.com site if you get an email. Do not click on any links in a suspicious email.
Best Regards,
Brian Boley