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.