Azure Cloud Service deployment targets have been deprecated in Octopus 3.1, and have been replaced by the 'Deploy an Azure Cloud Service' step.
Don't worry - you can migrate your Azure Targets to use the equivalent Azure Steps by following this guide.
Azure Cloud Service is available as a deployment target.
For more information on deploying Azure Cloud Services with Octopus using this deployment target please refer to deploying a package to an Azure Cloud Service. The same options and process is used by the deployment target and the deployment step.