Hello,
That product looks like it's using a DisplayLink DL-3900 chip and therefore uses the closed source driver:
http://www.displaylink.com/downloads/ubuntu
The older generation uses another driver.
We mainly use customer (companies buying chips from us) data to determine the user base: we don't design, manufacture or sell any end product, we're a silicon vendor making one chip going into the box... The seller decide what they want to support based on expertise and where they believe they will sell. I agree that there is a kind of chicken-and-egg effect in play. There are also many more Linux flavours than Windows or OS X / macOS and we are quite a small company.
The chipset is HDCP capable, however you won't get end-to-end HDCP on Linux. Communication on the USB link is nonetheless always encrypted for multiple reasons. I agree you don't have to bother with it.
For more specialized Linux questions, I'd advise posting on the Linux board my other colleagues watch as I'm reaching the end of my knowledge on the topic.
You could also have a look at the
latest Ubuntu driver release notes to learn about what is known limitations and future developments. (You'll notice the DL-6000 chip is mentioned).
Kind regards,
Alban