None of the examples cited are mitsvos. They are things that halacha allows that have become obsolete for reasons unrelated to halacha.
Economically, slavery is a bad deal (the owner of an eved Ivri got stick with massive bills for wife and kids, even though they didn’t do any work, and the eved Kaanani turned out to be highly inefficient once capitalism was invented several centuries ago – it seems that paying workers make them work a lot harder than an unpaid worker). Polygamy was never practical (most men have trouble supporting one wife and set of kids). Marrying first cousins seriously increases the risk of genetic disorders in the children, something unknown until the 20th century. Another factor is that due to improvements of transportation and communications everyone has a bigger pool of possible spouses (it no longer takes months for a letter from Jerusalem to western Europe, and travelling to Eretz Yisrae from Europe is no longer a life-threatening once in a lifetime effort).