I got screenshots!
Unless otherwise stated, the screenshot on the left is of firmware 4.23, while the screenshot on the right is of firmware 4.22. Don't mind the time displayed, as I'm very much a "night" person!
The most obvious change: window title replaced with a clock, and new bottom navigation. You'll also notice a bug here with the Pocket integration, that I've been meaning to report to Kobo for some time, along with other bugs relating to the Pocket integration and previews (samples).
The pop-out menus for things like Wi-Fi and brightness are identical to that of the previous firmware, so I won't show those unless someone wants me to.
Here's the new "My Books" view, with the new scroll bar and tab style on show. Not much has changed here besides those two.
(EDIT: I forgot to show Cover View, but not much has changed with that besides the new tab style and scroll bar)
Besides the new tab and scroll bar style, the "Discover" tab is identical to the "Kobo Store" option on the previous firmware. Unfortunately, I forgot to capture the menu on 4.22 before upgrading. As such, I won't be showing screenshots of those, unless someone here wants me to.
However, one thing I miss is the "Go to page" feature, which I usually used while scrolling through my wish-list. This screenshot is from firmware 4.22.
Instead, when dragging the scroll bar itself, the number of the page that will appear upon release is shown. As someone mentioned, both vertical and horizontal swipes work here This screenshot is from firmware 4.23.
As others mentioned, the Web browser is now inside a dialogue box, like when tapping to open Wikipedia articles, Web searches, or links within books, while reading.
The entire reading UI, and pop-up menus for the status icons in the top right, remain identical to the previous firmware, but may be changed in the next. Even the clock here is still in the top right, as opposed to the top left. That's why I won't be sharing screenshots of this unless someone wants me to.
"Beta Features" games and sketch pad do still work, and even though the window titles are no longer visible, they aren't too important to me, but I'd still like to see those, or it may just be me having not fully adjusted myself to using this new UI.
I have more screenshots available on request.