Using the latest version of the OpenAPI specification is always where you want to be when it comes to defining the surface area of your HTTP APIs and Webhooks. The latest, 3.1.0, as of this writing has significant JSON Schema and Webhooks that you will need to properly define the technical contracts for your APIs. Properly defined schema, as well as a low-cost HTTP event-driven layer is an essential aspect of API governance.
Version 4.0 of OpenAPI is currently being planned, but there are also additional minor releases being planned, so always stay in tune with what is the latest version, but also which version your services and tooling support. As of September 2024 you should be running on OpenAPI 3.1 and 2.0 of the specification should not be used at all. OpenAPI 3.1 should be the baseline of your API operations and governance so that you can properly validate requests and responses, but also include Webhooks as part of your technical API contracts.