On macOS, tap devices for L2 networking are not supported out of the
box. While a kext can be added to provide tap support, the kext
experience is not very good; Apple has strongly recommended against
their usage.
As a replacement that's documented and recommended, Apple introduced the
vmnet framework, intended for emulators and virtualization software
explicitly. This API requires macOS 10.10, with bridged network support
coming in macOS 10.15.
This introduces basic support for vmnet.framework in SIMH. I've tested
it by booting an emulated MicroVAX 3800 from an emulated InfoServer 150,
where it was able to reach OpenVMS 7.3 standalone BACKUP.