Polygamous marriage was a religious rite in Mormonism, presented as a requirement to get into the best part of Mormon heaven (as an aside, Mormonism divides heaven into 3 main tiers, ranging from “totally cut off from God and everyone you ever loved but still better than life on Earth” to “you get to be God”. And this is a simplification since the theology is wild). So when I say “practiced”, it’s just the usual terminology for when religious people do something dictated by their religion.

Technically, Mormon church leadership has disavowed polygamy, but like other people commented, they still teach that men can have multiple wives in the afterlife, and the current leader of the Mormon church has an… [interesting marriage at the moment (his second wife, so he fully expects 2+ wives after he dies.)] (https://mormondisclosures.blogspot.com/2013/09/same-tract-attractions.html). The exmormon subreddit has more discussion about this, if you’re willing to visit Reddit.