To expand on @databrecht’s point, the GraphQL API is effectively a versionless API for the time being. We’re following what is also called a Continuous Evolution approach. In practice, all of this means that there is no API version parameter at all, and that any new changes are always introduced in an additive way (in other words, there should be no breaking changes at any time).
That being said, we might introduce versions in the future though. Those might be tied to a (wide) limited time frame support, which should allow users to upgrade to newer versions.
In any case, it might be fair to add at this least this brief explanation about versioning within our docs as well for future references.