Hi everyone,
Here is a build of 2.4.2 (pre-release, not final) with HTTP2 support for macOS:
https://download.owncloud.com/desktop/testing/ownCloud-qt5.10.1-2.4.2.951824.pkg
To check if it uses HTTP/2, you can click on the 
icon in the UI, the first item should be "HTTP/2" if your server supports it too..
Please report back any issues you might have 