Google is building its own DeX: First look at Android's Desktop Mode

Pretty sure Windows Phone did this over a decade ago. I mean, say what you want about Windows Phones, but yeah, this was a thing.

I'm not aware of any Windows Phone implementation like this that existed commercially. Can you point me to it?

The first modern thing like this that I can recall is the 2011 Android-based Motorola Atrix phone[1] that presented a DeX-like desktop (well before DeX!).

It used an Ubuntu-based desktop. It was really, really good, but never got traction.

1. https://en.wikipedia.org/wiki/Motorola_Atrix_4G