Almost every scriptural religion closed its canon at some point and then argued about how to interpret what was inside it. The Latter-day Saint movement did not. The ninth Article of Faith, published in 1842, states that God has revealed, does now reveal, and will yet reveal many great and important things, and the Doctrine and Covenants is the standing evidence: a book of scripture assembled from revelations received in the 1820s and 1830s, added to since, and formally open at the end.
The additions are few and they are documented. Two sections were canonised in 1976, one of them Joseph Smith's 1836 vision of the celestial kingdom and the other Joseph F. Smith's 1918 vision of the redemption of the dead. Official Declaration 1, the Manifesto of 1890 ending plural marriage, and Official Declaration 2, the 1978 revelation ending the priesthood and temple restriction, are both canonised. Each was presented to a general conference of the church and accepted by a sustaining vote before it took its place, and that procedure is itself part of the doctrine: revelation is announced, not imposed silently.
An open canon has a cost the movement lives with. If God can still speak, then a practice taught as doctrine in one generation can be ended in the next, and the church must then explain what the earlier teaching was. That question is live for plural marriage and for the priesthood restriction, and the answer the church gave in its 2013 Gospel Topics essay, that leaders had advanced theories which the church now disavows, is a direct consequence of holding the canon open.