Most samples here haven’t been halachas. Polygamy was mutar, but it certainly wasn’t a mitzvah. I don’t know that it was even a lehatchilah.
Tevilas Ezra was a real halachah (I think) that Litvaks no longer do.
Leaving over the peah of your field (not hair) for the poor is a halachah with it’s own Mishnah, but we don’t do it anymore.
Does anyone really matir not washing before soaked vegetables, or is it just a “the masses are meikel and we do not protest” situation?