The “channel” parameter still works like it always has, but remember that the /streams resource only returns live streams, not offline ones.
So you’d still want to use the call just like you describe, and treat any stream that is not returned as being offline.
There is still rate limiting, and you should always include your client id (See https://github.com/justintv/Twitch-API#rate-limits)
Also remember to include the “api_version” parameter for the resource, as it may save you some trouble in the future (the latest is version 3, which it defaults to if you leave it out)