That's like saying that using "yeah" and "fuck" make you sound like you come from a bad neighborhood. Totally untrue
Replying to a question with "ouais" is rude in French if you're not chatting with your friends.
Some Verlan words are still in common use in the "banlieues" (i.e. rough neighborhoods), like saying "muff" instead of "femme", "tromé" instead of "métro", "chelou" instead of "louche", etc...
But there are many other slang words in use. A lot of borrowings from English ("cash") or Arabic ("flouz" meaning "money") are used too.
It's probably hard for an English speaking tourist to hang out in these areas. But if you want to get a "feel" for it, I would recommend watch "Spiral" which are some stereotypical "banlieues" scenes. But I suppose it's better than nothing... or going to the "four" to buy drugs.
Flouze isn't a recent borrowing. It came when Algeria was still French, probably through the Army. Like toubib, caoua, chouïa etc. It's not immigrant-specific.
There are new borrowings from Arabic, and they're usually very noticeable and have strong social implications. Similar to the Spanish used by Latino-Americans in everyday English.
"tromé" might be a bit too young-sounding for general use, but nothing to do with politeness or social class. The well-off Parisian youth love to use these words.
But 'ouais' is not just familiar, as is in "perfectly fine with people you know", it is lacking manners. Generations of French mothers, teachers, etc. have spent a lot of time scolding children who answered them using "ouais" instead of "oui", as another commenter mentioned. In general it is fine between friends or colleagues, though.
'Putain' is very rude outside of very familiar banter among friends and colleagues.
'tromé', and verlan in general, is quite low social class.
If people here on HN want to learn French then it is obviously useful to know these words but they should not use them. You never get into trouble for speaking properly...
Anywhere else, it is acceptable, don't get offended if someone says those words to you. I have literally heard it used in business meetings with the executives of top funds and companies.
Language in business can be quite spicy but you don't use that language to address your local baker if you have the most basic manners (granted, many people don't).
But those words are, broadly speaking, perfectly acceptable in many situations. Two 20-something or 30-something friends will use "ouais" (and "yeah"), "putain" (and "fuck" or "damn") and even potentially "cette meuf" (and "that dude") without any issue. You wouldn't use those in professional settings in French or in English.
Isn't that exactly what I'm saying? Using those terms between friends (age is irrelevant and those terms are not just for the 'young') may be OK but you would not use them to even talk to a stranger in the street or in a shop if you have basic manners.
Children are taught not to say "ouais", or not to say 'tu' to adult they don't know, etc. because it is bad manners to.
If I'm asked if I want my baguette on the more crunchy side and I reply "ouais" this is bad manners.
Long story short, most of everything else discussed in this thread is not appropriate. If you address women as meuf (hoe), your friendship and romantic prospects will dwindle like an ice cream under the sun.
If that's how you talk in an interview or on a date, you're out in less than 5 minutes (they may wait the end of hour to show you the door, they are not monster -unlike you-).
Also, verlan is used all the time by everyone under the age of 30. Not all of it; some terms are rarer than others and are, as you say, associated with rough neighborhoods. But it's an extremely common way of speaking.
Also, using slang isn't as frowned upon as it can be in America. Again, idk your experience but mine is that people swear left and right in casual conversation without anyone giving them a side glance. Of course in formal contexts such as addressing strangers in the street or greeting your baker you don't want ot, but the moment familiarity sets in you can be sure swearwords are going to pop all the time.