This is a very interesting topic for foreign learners. In Russian, as in any language, there are some slang words that exist only in certain subcultures, such as those of criminals; other slang words come out of the подполье ("underground") and become fashionable in "normal colloquial" speech for a while but then quickly go out of fashion; still other slang words become a permanent part of the "normal colloquial" lexicon to such a degree that they perhaps shouldn't be called slang any longer.And there are gradations of slang
LOL! Russian 'mat' is a topic of "вечнозелёный" fascination to us foreigners! True, it eventually becomes boring for learners at a more advanced stage, but every September there will be a fresh wave of university students in a first-year Russian course who urgently want to learn all the cuss words! (As well as those who want to show off their ability to say "Yabb tevye match" in perfect Russian.))))I am not talking about 'mat' here. It is a completely different topic. And it is a topic that very soon gets too boring to deserve a special section
Anyway, although slang doesn't necessarily need its own section, perhaps there could be a "sticky" thread for просторечье, сленг, жаргон, и ненормативные лексике, into which people could add significant examples that occur in other threads. And foreigners could ask, "I found this word on the Internet and I think it's slang -- is it worth my time learning this word?", so natives could offer opinions on whether the word is in widespread currency or not.