Whether you’re writing Asynchronous or Open APIs unless you’re doing it pretty much constantly, it is useful to have links to the specific details, to quickly check the less commonly used keywords, or to check whether you’re not accidentally mixing OpenAPI with AsyncAPI or the differences between version 2 or version 3 of the specs. So here are the references I keep handy:

• API Handyman’s excellent OpenAPI Map and his spec navigator for Async and OpenAPI
• Async API Schema (v3) and v2.6
• Open API Specification (3.1)
• JSON Schema
• YAML Schema
• Common Mark (as allowed in AsyncAPI for formatting text)

There are some useful ISO Specs for common data types like dates. Ideally, if you’re working in a specific industry domain, it is worth evaluating the industry standard definitions (even if you elect to use entire standardized objects). But when you’re not in such a position, it is at least work using standard ways of representing data—it saves on documentation effort.

• ISO Message Catalog covers a wide range from Swift to Telecos (emphasis is on financial-related standards)
• ISO 3166 – Country Codes
• ISO 639 – Language Codes
• ISO 8601 Date & Time

Not a standard, but still an initiative to promote consistency back by the likes of Microsoft etc, so could provide some insights/ideas/templates for common data structures – https://schema.org/

There are, of course, a lot of technology-centered standards such as media streaming, use of HTTP, etc.

These and many more resources are in my Tech resources.