Yeah the era of non-Ethernet/Wi-Fi NICs died off decades ago with the last ADSL cards. Nowadays I'm not sure if OSes even support creating drivers for anything non-Ethernet (especially where to provide the config UI for your non-standard protocol).
What I've seen done recently to work around this is to combine your custom chip with a standard Ethernet NIC on the same board. The computer just sees an (off-the-shelf) NIC that's always connected, and all configuration happens via IP by browsing to a specific private IP (this kinda insists on NAT though).
Linux would support it for sure. It even still has support for several old NICs (it was only the other day I saw a news item about some old protocol from the early 90s finally being removed). But I can imagine no one wants to develop a new such driver.
And if you want to sell to consumers you need Windows and Mac support, and then it easier to just adapt to existing interfaces.
However, I wonder why it seems G.hn is only available in the form of adapters, and not as e.g. a PCIe NIC.