Hi! I've tried it for a week now. It's still impractical if the only place you can 'go backwards in a thread' is down at the bottom of a page (on mobile).
Considering that obviously there's a loss of functionality there, I don't get what you're trying to say here. Old forums: We can go backwards at the top of the page. New forums: Scroll down to the bottom of the page before you can select a page number. How is this not a loss in functionality? How does the forum upgrade have nothing to do with this?
"Jump to new" also doesn't function as it should. I refresh the page (to make sure I'm up-to-date) and click Jump to New. It jumps to the bottom of the page and there it says: 1114 of 1116. Shouldn't it have brought me all the way up to page 1116?
What I described was purely on mobile, click the date and not the thread title and you'll be brought to the most recent post.
I don't often browse on mobile but from what I remember the page switcher was only at the bottom on XF 1.0 as well, but I could very well be wrong there. Otherwise, I do get that it's a bit inconvenient, but it doesn't take very long to just scroll to the bottom of the page on a phone. If you're browsing on a non-touchscreen device then it would certainly be a pain, but so would using the web in general. To my knowledge, as simple as this feature may seem, it's not something that could easily be implemented if at all.