The clusterctl CLI is developed in lock-step with Cluster API. We strongly recommend using the latest released version for the series you're using. For example, if you're managing clusters with Cluster API v0.3.7, the clusterctl version should be >= v0.3.7.
When this package is used as a library, we do not currently provide any compatibility guarantees. We will make reasonable efforts to follow a typical deprecation period prior to removal, but breaking changes can happen when necessary.