Your user account is 'owned' by the ArcGIS Online organization. To revert to a 'public' account you need to:
- Sign into the ArcGIS Online organization as an administrator.
- Remove all content and groups owned by the user
- Remove the user from the organization (ensuring 'Esri Access' is enabled so that the account is not deleted entirely.
The issue is that you only have one user in your personal subscription. Meaning you can't sign in with an additional user to remove the other one. To achieve this, you need to contact Esri support, but they may also have some issues with your subscription being expired (support is limited for personal subscriptions and further limited if you don't have active maintenance).
For what it's worth, this account is an expired personal user account. It doesn't matter for me. I use other accounts for work. I can still sign into ArcGIS Online but I don't need to with this account for any reason. I can go back and renew my personal user subscription if I feel like.
