As of 2019, are there alternatives to the official Skype client?

Now there is a Web UI. This does not provide the whole features, but at least it is not a resource-hog. https://web.skype.com/

As you have mentioned, Skype's protocol was never published, so no alternative clients are available.

There's a IM Client I know called Miranda IM, which integrates to Skype but requires it to be running in the background.

If you're into gaming, you can try Overwolf or Xfire, which both integrate to Skype and other Social Media sites (ex. Facebook).