While T232485 is getting hammered out, we need to have an API namespace so we don't conflict when the actual API is released.
Barring other options, I'd like to use "/v0/<rest of path>". It's clear that it is an early version, per semver, and subject to change. When we finish T232485, and if we end up using "/v1/" as the Core REST API prefix, it will be a natural upgrade from v0 to v1.