Swagger has been available as of Mendix 9.17: https://www.mendix.com/blog/mendix-release-9-17-a-lot-of-activity/#data-hub-connector-kit-2-0-openapi
All contracts and documentation are available in the running app starting from the /api-doc/ path, e.g., http://localhost:8080/api-doc/
Swagger preview in Studio Pro was recently added to give developers a quicker preview of what the odata api looks like for a regular REST client developer.