If I understand this correctly, they do the same thing here if you look at the main forum page you will see Full Releases (Subforums) and Full Releases (Old Layout). The old layout is just a search of everything within all the subforums.
So if your parent forum is id 33 then
https://vborg.vbsupport.ru/search.ph...days=1000&f=33 will search and display all of the posts within the subforums of that parent. the path will be
http://www.yoursite.com/forum/search...y&days=1000&f=33 and change 33 to your forum id, then make a new forum with that link in it and it should do what you are asking, at least in part.