Some new APIs in macOS, like low-latency HTTP live streaming, are available only to apps distributed via the Mac app store. This is not how an open platform works. On an open platform, all client-side APIs are expected to be available to all apps, not just those specifically blessed by the platform vendor.
I’m not talking about APIs for Apple’s cloud services like in-app purchase, Sign In with Apple or Game Center. These are server-side, and any restrictions for using them don’t make macOS a closed platform on the client-side.
But HTTP live streaming is a protocol that anyone can implement on their server, so the client-side API should be available to all apps.
There are many other APIs that are not available to developers who refuse to pay Apple $99 a year: Network Extensions, which lets your app connect to a VPN, manage Wifi connections, create a content filter, or proxy DNS requests. Custom Network Protocols, which lets your app build protocols on top of TCP or UDP. Associated Domains, which allow your app to be associated with your domain to access shared passwords or continue an activity in your app. Endpoint Security Extensions, for things like antivirus software. DriverKit, which lets you write drivers that run outside kernel mode. None of these APIs are available for you to use unless you pay Apple their annual tax. That’s not how an open platform works — you shouldn’t have to pay a rent to the platform owner, any more than you need to pay a $99 fee to the W3C to build a VPN server.
Moving beyond APIs, Catalina’s system volume is read-only. You can make it writable, but it reverts to read-only on reboot. An open platform can have defaults that are right for 99% of users, and the knobs to change them can be hidden, such as having to run a command on the command-line, but once you do so, the system should respect your decision and not keep overriding it.
In conclusion, the Mac is no longer an open platform. I won’t buy one anymore for personal use. I’m writing this from a Windows laptop, the first one I bought since using the Mac as my primary platform for almost a decade.