This is a fairly technical meme. Userspace is not the same as user preferences, and in this case refers to application compatibility. Applications written by third party developers (i.e. not the creators of the OS itself) are almost always in userspace and not kernelspace.

Windows and the Linux Kernel devs go to great lengths to ensure that they are backwards compatible, sometimes to a fault. For example, there are certain bugs that are left in place and not fixed because some applications have adapted to the bugs and now rely on that behavior. If the bugs were fixed, suddenly those apps would break and the developers of those apps would need to create an update. That’s complicated or even impossible if the app developer has been out of business since the 90s/dies/is locked up in legal limbo/etc.

You can still run games and other software from the 90s on Windows 11, but there is software from the 2010s that won’t run on the latest version of macOS because Apple doesn’t give a fuck about maintaining backwards compatibility (breaking userspace).

To keep it simple one could say userspace is what the average user faces. Eg the audio engine, the desktop shell, the window tiler,… So basically the things that make your apps and make them work.

Windows 11 has the ability to natively run XP software iirc because they don’t break the underlying systems, they rather extend them.

macOS in the other hand changes stuff all the time and if a developer doesn’t catch up with that, the next macOS release will break the software.

That’s why I couldn’t include Arch haha

They update stuff and if devs don’t update their software, usually within 2 macOS releases the software stops working. A nice example I once had was MS Office

See my other reply here for a breakdown of what it means to break userspace. One recent example was when Apple removed support for all 32-bit applications in macOS 10.15 Catalina. It’s something they do quite regularly with the attitude that app developers can either update their apps, or their apps will simply not run on macOS going forward.

It’s not necessarily a bad thing to force developers to update their apps in this way, but it does mean that macOS does not have backwards compatibility at nearly the same level as Windows or the Linux kernel. If you care about running older software (say as a business with a critical application that would be too expensive to replace/update, or to play an old game on your modern machine), macOS is likely a non-starter.