Not in my circle. Usually, threatening to sue is enough. I know people who increased their severance package because they said that "they wanted to talk to the lawyer from their legal insurance before accepting any severance package."

Of course this doesn't work if you have a work council, usually the work-council negotiate a severance package algorithm (= fix_amount + tenure * amount), and this is usually un-negotiable. This makes sense, thanks to collective bargaining, it's most likely one the best deal you could have gotten. (Even though libertarians will flock-in and start going "How do I know, that I, as a highly performing individual couldn't have negotiate better!?! This seems unfair!")