Hello, thank you for reaching out to Microsoft Q&A.
According to the Microsoft documentation, only a user with the Organizer role can remove members or child accounts from a Microsoft Family group.
If the family organizer account is inaccessible, you must try the account recovery options provided by Microsoft.
However, in your situation the Organizer account has been inaccessible for more than 15 years, where you will not be able to even use the account recovery form to regain access to that account. Even if you contact Microsoft Support directly, they will provide the same answer I’ve shared above.
In this situation, I’m sorry to say that the account is likely lost and there is no way to recover it.
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