How official is TNO 486958, 2014 MU69's new name Arrokoth?
The International Astronomical Union describes this minor planet naming process:


Minor Planet Center assigns provisional designation
MPC assigns permanent designation
Discoverer suggests name
IAU working group reviews name
MPC publishes name



  Accepted names become official when they are published, along with their accompanying citations, in the Minor Planet Circulars, issued monthly by the Minor Planet Center.


The last page of the Minor Planet Circular published 2019-11-08 says:


  (486958) Arrokoth = 2014 MU69
  &nbsp; &nbsp; Discovered 2014 by M. W. Buie and the New Horizons Search Team.
  &nbsp; &nbsp; Arrokoth is the word for "sky" from the Powhatan language of native people from the Chesapeake Bay region.
  Institutions in this region played a prominent role in facilitating the discovery and exploration of this ancient and distant object.


so it's as official as it gets. As with these near-Earth asteroids:


(99942) Apophis = 2004 MN4
(101955) Bennu = 1999 RQ36
(162173) Ryugu = 1999 JU3


we can expect (486958) Arrokoth to displace 2014 MU69 as the prevailing identifier in new publications.