What a terrible name ('HTTP API'), it's taken me some time to figure out what this is about...

Blog post on the announcement last year:

https://serverless.com/blog/api-gateway-v2-http-apis/

And now I see that the APIGW page does mention it:

https://aws.amazon.com/api-gateway/

But it's really not searchable. Maybe it doesn't matter, and is just an implementation detail of Serverless? I don't know, I still don't think the 'choice' of HTTP, REST, or websocket is well-named.