The accept header is a mechanism for communicating formats you support; it doesn't do anything to address the problem of managing change to supported versions, which is a dev process issue, not a technical issue.

Feature flags, just enable it.

When all applications are adjusted, the accept-headers request a protobuff format in return.

=> Propagated everywhere except when a js ajax calls happens to the api-gateway.

> The accept header is a mechanism for communicating formats you support; it doesn't do anything to address the problem of managing change to supported versions

That assertion is not true. Media type API versioning is a well established technique.