I suspect that, if challenged, a judge would equate reasonable notice periods with the minimum-severance terms set in law, at least for employment that lasted o

I suspect that, if challenged, a judge would equate reasonable notice periods with the minimum-severance terms set in law, at least for employment that lasted only a few years. So the 6-months clauses you see are probably a bit like noncompetes: everybody knows they are not enforceable, but companies still enjoy the chilling effect they produce.