Proprietary systems are systems that are owned by their developer or vendor, and the company does not have access to the source code or other necessary information to upgrade or patch the system. This can make it difficult to remediate vulnerabilities in proprietary systems, as the company may need to rely on the vendor to provide a patch or update.
In this case, the two critical systems cannot be upgraded due to a vendor appliance that the company does not have access to. This suggests that the systems are proprietary, and the company is unable to remediate the vulnerabilities without the vendor's assistance.