I noticed the back to top button was missing from the most recent dev builds, but I hadn't parsed exactly what that meant. As I've started to use 4.0 more getting a feel for what I will need to do switching across, its become apparent that this was a problem for us.
The developer update says it was replaced by the course index, but I don't believe that to be true.
Example one: A long course:
This is a long course and I use tile/grid formats to make it work in 3.x. Each section is a single screen-full and that works great for grid/tile. However, as you can see with 4.0 un-modded, at the bottom of this course, I'm both a long way down both the main viewport and the course index and have no quick way to get back to the settings. I know I can just press the home key on my keyboard, but if Im scrolling using the mouse, I'd like a mouse based means of return.
Example two: Course settings
Back to top wasn't only useful for course pages, but also for other
resources on the course pages itself. Here, the same course settings
page and there is no quick way to return to the top. The Index feels almost entirely pointless on this screen, with the only use I can see is jumping back to one of the course sections.
Example three: A long course resource
Here I have a long page resource (ignore the generico filter stuff, I
haven't installed generico, or any other plugin, on my 4.0dev to keep
things simple). Once again, this long resource would benefit from a back
to top. At least here, the course index is useful.
The back top top removal only really makes sense on the course index page and not always even there. There are too many other places it remains useful (another example is the site settings page).
I do wonder, however, if this is actually a symptom of a a different problem: As we scroll down, we lose access to the relevant navigation. What if instead, the relevant navigation was stickied to the screen as we scrolled down: