I used this feature a lot in trading threads on 2.0 to find if anyone's selling a particular thing, so I really hope searching one section or topic will be possible again.
I'm able to get to it directly on the mobile site just by pressing "search" from the menu. As for the regular site, the only method I've (accidentally) found is to search for a single letter. An error page will come up, but the advanced search page will be there underneath it.