Almost everything really big and complicated has been upstreamed by now (binder, ashmem, EAS, many Qualcomm drivers, ...). Some commercial phones can boot Android from a mainline kernel including the Pixel 3 and the Poco F1.
Anbox requires your kernel to have ashmem and binder, which are included in the ckt (Canonical) kernels or they can be loaded by DKMS if you don't have them already.
