Discontinuance
Hyrum Smith remained Assistant President of the Church until he and Joseph were killed on June 27, 1844. The death of both the President of the Church and his supposed successor, the Assistant President, helped contribute to the leadership confusion among the Latter Day Saints which led to the succession crisis of 1844. In most of the separate denominations that resulted from the crisis, the position of Assistant President of the Church was discontinued.
In The Church of Jesus Christ of Latter-day Saints, the largest of the Latter Day Saint denominations, the lack of a current Assistant President of the Church has been explained in the following way:
After Oliver Cowdery fell from his high status, Hyrum Smith the Patriarch was chosen by revelation to succeed to the position of Assistant President and to stand as a joint witness with the Prophet of the truth of the restoration. ... When these two joint Presidents of the Church sealed their testimonies with their blood, the full operation of the keys of the kingdom rested with the Twelve, and Brigham Young, the senior apostle, became the ranking officer of the Church. Since the kingdom was then fully established and the two witnesses had left a binding testimony, it was no longer necessary to continue the office of Assistant President. Accordingly, the office is not found in the Church today.
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