A newbie jail would work, but it would still allow the beginners section to become cluttered with the potential spammers.
It could be a sub board of Beginners and Help, though them being essentially shadowbanned until whitelisted would also work.
A "newbie jail" could work but only if it wasn't x amount of posts and more a case of meritable posts to get out although with that you would also have to have a dedicated mod and merit source.
This is one of the reasons why it was removed in the first place I believe because people just spammed rubbish for the x amount of posts over x amount of time then moved out. Making them get the merit first would probably be too difficult and time-consuming for them and some people would just get trapped in there indefinitely or quickly give up out of frustration.