One of the quieter features of Android 9 is that they are reducing the API surface of Android by explicitly black-listing and white-listing various APIs and fields that previously required reflection to access. None of these APIs were documented, but none of them were explicitly "off limits" before Android P. That suggests that the life of Java as Android's primary language might be really coming to an end, even thou…
Apparently you haven't been paying attention, declaring off limits APIs started with Android Nougat for NDK developers. Because Linux APIs were never really intended to be called by NDK developers, only the POSIX subset and Android public native APIs. Likewise with Java, which is only a subset of proper Java. In any case, it might relate more to an eventual future support of Java 9 module system, where private really…
That's not even slightly true? Android doesn't pretend to be anything but a Linux system, with all the Linux details shining through in all their glory.
There is a seccomp policy that blacklists some syscalls, but it's only 17 out of 271 for ARM64 and it's things like setuid or reboot which you won't have permission to do anyway. Other than that you're completely free to go nuts with raw syscalls, you're in no way, shape, or form stuck going through a POSIX layer.