Do you think any Halachas that the Shulchan Aruch or the Chofetz Chaim tell us to do are no longer done? Going through Sefer Chofetz Chaim I haven’t found any such examples.
As far as your examples, Sefardim do Yibum. Teimanim and some Sefardim (i.e. Moroccan) have polygamous marriages. Avadim is certainly muttar if a country allows it. And Chasidim, especially Rebbishe families, often marry first cousins today. As do numerous non-Chasidim. Chasidim everywhere (including in the U.S.) as well as Yerushalmis and many other Litvaks in Eretz Yisroel often marry before age 18.