Place Shadab's code at the bottom of the find_new_threads template. When there is something new the sidebar will show. FYI, if there is nothing new to display, the sidebar will not show as only the breadcrumb templates are loaded.
That wouldn't work because the data required for those templates usually won't be available on the what's new page. For the "dark_taigachat" and "yilmaz_top_threads" templates, you'll need to contact the respective addon author.