When API requests are made without a version prefix, the daemon treats the behaves as if the request specified the newest API version supported by the daemon. Any client software which makes unversioned API requests can therefore be subject to the semantics of API versions newer than the version it was tested against, potentially leading to breakage and compatibility issues which could have been avoided if the client was explicit about the API version it wanted to operate under.

I propose that we freeze the API version implied by unversioned API requests so that unversioned API clients tested against existing daemon versions will continue to work across daemon upgrades. Any client which wants to make use of functionality introduced in API versions introduced after the freeze would have to opt into it by versioning the API. The unversioned API would effectively be deprecated in that software using it would continue to work as it always has, but no new functionality would be available without first migrating to a versioned API.