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.