Im a practicing Mormon that grew up in Utah. Joseph Smith definitely practiced plural marriage
There have been break off groups that still try and practice it My dad was a high Mormon Church official and he would get letters from Church headquarters of certain people practicing plural marriage in our area to excommunicate
There once was a pamphlet written by Joseph Fielding Smith it was called, The Origin of Plural Marriage and Blood Atonement. In there he gave eye witness accounts of his family and his great uncle Joseph practicing plural marriage
heres a quote from the pamphet by JFS himself
THE UTAH VISIT.
In connection with this, let me call your attention to
your visit to Salt Lake City some three years ago. At that
time you met President Lorenzo Snow, a man whose veracity
cannot justly be questioned; you heard him bear his testi-
mony to the effect that he was taught that principle by the
Prophet Joseph Smith, and that the Prophet declared to
Lorenzo Snow that he had married his sister, Eliza R. Snow.
You met and conversed with Lucy Walker Smith, and she
told you that she was married to the Prophet Joseph Smith on
the first day of May, 1843, in Nauvoo, Elder William Clayton
performing the ceremony. You met Gatherine Phillips Smith,
who told you she was married in August, 1843, in Nauvoo,
to the Patriarch Hyrum Smith, his brother Joseph the Prophet
officiating in that ceremony. You will remember that the
first wives of both these men were living at the time. I hardly
think these testimonies have passed from your memory in so
brief a time. I am personally acquainted with these women,
and know that they are truthful and honest — honorable
women, whose testimonies should be believed.