I recently subscribed to Curse Premium (though I think I had some kind of lifetime "Premium" granted to my account by Kael way back in the day for helping test stuff...).
I subscribed (more officially) under the impression that I would be able to sync addon settings - especially since clicking the "Sync" button in the macOS client reveals a modal prompting me to subscribe.
After subscribing, the "Subscribe" prompt was still there (when selecting "Sync"). As it turns out, this is a "Windows Only" feature - something I had to (unfortunately) discover via the Curse Support site; the note on the article itself is buried at the very bottom.
If there are no current/forthcoming plans to sprint this feature or it is buried so far down the backlog that it is not likely to see even an edge/alpha release any time soon, please remove the button from the macOS/OS X client.
That said, I encourage everyone involved to take the plunge and make the effort to achieve parity across platforms. I'd be willing to allocate 5 hours per week in testing, if that helps.
I'm extremely surprised the OSX Curse Client is still functioning for you, to be honest. It hasn't been updated in years. You should be using the Twitch Desktop App, which does have feature parity between Windows and Mac, and is the replacement for the Curse Client.