I attended the special event last week (besides: it was a fine showcase for all the overwhelmingly great new features!), and since then I was playing around with my 160+ topics RoboHelp 2019 project in the 2020 Trial version. Great fun, even though I didn't even test the new Review process and creating sophisticated PDFs with that new tooling!
Of course, I started the purchasing process already within our company (worth every penny!!), and also for shifting RoboHelp to macOS in a private license of TCS which I'm about to make use of for my upcoming Master thesis.
By the way: I also was wondering how to utilize the spaces on the left and right in the top-menu skin, where the panels had been previously.
If we change to that skin later on, it would be for the greater topic width that allows for large tables and schematic drawings to be visible without scrolling. By now, on my first approach of generating an output with our data from Rh 2019, the topic width stays the same leaving the space of the panels just blank.
Did I do something wrong in the skin's settings?
I haven't done any work with the new skins so I can't help you with that aspect at the moment.
I would be very surprised in 2019 would support those skins but there's nothing to stop you creating a project and trying. I wouldn't go that route though as if you hit any problems, it would not be supported.
Okay, now that I've tried it in a nearly blank project I can say: NOPE, doesn't work. Crashes immediately.
There are some structural changes in those skins and so would not open in 2019.
Thanks for updating the thread to save others trying it out.