Arduino Moving from Mbed to Zephyr

Typically, when your libraries don't compile on newer boards it's because the author used register-level operations and features that aren't portable across processor architectures. The more popular features don't suffer from this because they end up being ported. e.g., PWM is extremely valuable, so PWM implementations exist on all arduino architectures and the AnalogWrite() interface at the Arduino abstraction level is the same, so you never need to deal with it.

But a programmer may have written a library version that's optimized for a particular device such as a MegaAvr variant, maybe using PORT writes instead of the slower digitalWrite() & digitalRead() abstractions. Including that library on a Blackpill will cause compilation problems. I think this problem exists with some libraries that do high-speed ADC capture.

It sounds like they're saying that you don't need to worry about the core Arduino libraries, so 90% of projects are probably fine.