1. Polygamy, although permitted in the Torah, is not required, and has been banned by rabbinic decree.
2. The form of slavery alluded to was actually the biblical punishment for theft. A thief was sentenced to repay double the amount of the theft. If he did not have the money, the court forced him to work until the debt was repaid. As with polygamy, employing a slave is not a requirement BUT AN OPTION. The reason this method of punishing theft does not exist today, needs no explanation--use your imagination.
3. The prohibition against shellfish as not kosher is still in effect.
Rabbi Gershom's ban on polygyny (polyandry has always been strictly forbidden) was in effect for one thousand years and only for 'Ashkenazim. It has since expired. Now the only law against polygyny is "the law of the land," which, according to the Talmud, must be honored.
As a matter of fact, since Rabbi Gershom's ban has expired there have been cases (as I understand it) of Jewish men whose wives have gone into comas who, after obtaining a secular divorce (thus satisfying "the law of the land") have remarried without a Jewish divorce, meaning that Halakhically they have two wives.
2. The form of slavery alluded to was actually the biblical punishment for theft. A thief was sentenced to repay double the amount of the theft. If he did not have the money, the court forced him to work until the debt was repaid. As with polygamy, employing a slave is not a requirement BUT AN OPTION. The reason this method of punishing theft does not exist today, needs no explanation--use your imagination.
There are two kinds of slaves: Jewish slaves and `avdei-Kena`an (Canaanite slaves; not actually Canaanites [since they were to be exterminated] but called that since they were non-Jews). Only the Jewish slave was an indentured servant or was sold into slavery to repay a debt (which is much more humane than being locked in a prison like an animal and subject to violence and sodomy by one's fellow inmates, btw). Canaanite slaves were property (according to Parashat Mishpatim) and were handed down from generation to generation without being freed (except as compensation for certain injuries). The Torah also requires the enslavement of certain non-Jews in certain very specific war situations.
All Torah is immutable, and when Mashiach comes (may it be soon) all Torah will be restored.