Universal Containers (UC) has a recruiting application using Metadata API version 35, and deployed it in production last year. The current Salesforce…

The Metadata API validates deployment packages against the API version specified in the request, not the version of the destination org. The new field was introduced in API version 36. Since the deployment script specifies version 35, the validation service checks the Apex page's metadata against the schema of version 35. In that version, the new field does not exist, causing a validation error and failing the deployment. The component is considered invalid for the specified API version.

A. New features in a later API version are not backward-compatible; older API versions are unaware of them.
C. The platform's current version is irrelevant; the deployment is validated against the API version specified in the deployment package/script.
D. The Salesforce platform maintains support for many previous API versions to ensure backward compatibility for existing integrations.

1. Salesforce Metadata API Developer Guide. "Introduction to Metadata API," section on "API Versioning." "When you deploy changes... the components are processed according to the behavior of the specified version... If you’re trying to deploy a component that was introduced in a later API version than the version you’re using for the deployment, the deployment fails."

2. Salesforce Metadata API Developer Guide. "deploy() Call," section on "DeployOptions." The documentation specifies that the API version used for the deployment determines the set of valid components and attributes.