32bit PPC ran on intel macOS, which is probably harder (it's basically a problem of how you map 32 registers onto 8).
The difference is that after a while Apple is willing to deprecate things. Versus a lot of open source projects that don't guarantee any stability at all, even across minor version increments. That's the problem that makes them dangerous for platforms like macOS and iOS to expose them as public API (intentionally or unintentionally - my understanding is for instance that the system openSSL was accidentally exposed publicly - and that took most of a decade to remove from the OS, e.g. most of a decade of shipping an increasingly old Frankenstein branch of openSSL)
The difference is that after a while Apple is willing to deprecate things. Versus a lot of open source projects that don't guarantee any stability at all, even across minor version increments. That's the problem that makes them dangerous for platforms like macOS and iOS to expose them as public API (intentionally or unintentionally - my understanding is for instance that the system openSSL was accidentally exposed publicly - and that took most of a decade to remove from the OS, e.g. most of a decade of shipping an increasingly old Frankenstein branch of openSSL)