For various reasons some "private keys" are public.
Example private keys are part of standards, software test suites and public documentation. Sometimes such example keys are used in production due to a lack of understanding how public key cryptography works.
No matter why a private key is public, it is obvious that a "public private key" should be considered as insecure and compromised.
The "Kompromat" repository collects "private keys that have become public". We use this as the basis of our blocklist.