1. It's still there - however, it is not visible in 'mobile' view.
It's right above the list of replies for a thread (or as you scroll down the page, right below the OP) on the right hand side. The following link should take you there on a sample thread if you're viewing in 'standard' or 'classic' view:
I'm pretty sure the feature still works in mobile, the links just aren't visible. If you want to toggle back and forth between highlighting the 10 newest replies or the replies posted after you mark a forum, you can add &togglemark=1 the end of the URL for an OP when not in 'view all' mode. For example, when viewing just the OP of thread:
That last URL is not a working link in order to prevent people from changing their settings if they happen to click on it without realizing it will change the preference for what gets highlighted by the mark function. It should be easy enough to copy and paste the URL into a browser's address bar for anyone interested (or to just add the &togglemark=1 portion to the end of any OP in single-post view mode).