I contacted the Dutch Philips support center about USB ethernet not working in ATV 14 and received the following answer:
At this moment there is no official confirmation if support for USB ethernet adapters will be added again to a future ATV 14 update. I can assure you that your feedback will be forwarded to the relevant team for further investigation and consideration in future releases.
And then the funny part:
In the mean time I want to ask you if you are using a compatible ethernet RJ45 cable with your TV.
So it may be useful if as many people as possible complain to Philips about this.
Anyway, I hope that this was a decision made by Philips/TP Vision and not one that was made by Google when developing the kernel for ATV 14. Because if it was Google who decided to remove this feature, the chances of getting it back are probably slim to none.