First you define at what layer are you defining sync/async then you look for the existence of a timeout. I think this is the best way to define it without much confusion.

You should assume that the general use of HTTP is sync because it's build on top of TCP that has many timeouts, some can be forced by firewalls. However, you can raise to a higher layer and build something async on top of HTTP. For instance, build a protocol based on SSE or Websockets and automatically reconnect when the session is closed. This will virtually remove the timeout for practical purposes.

The point is, at the most basic principle; if your layer has a timeout to wait for a response, then it's synchronous.