The center channel (if a true center not just a mix of L+R) seems to have been lost
The “centre” music channel, as it was originally presented, does exist on the magnetic stereo releases. I was given three discrete audio tracks— left, centre, and right— when my print was captured. It’s why the 2000 and 2010 home video releases also have discrete centre channels. Disney didn’t artificially generate that with some plug-in or standalone software, it’s always existed, and it does contain information unique to that channel; it’s not just a cheap sum of the left and right signals.
For example, apparently the church bells at the end of the Bald Mountain came from the back of the theater, for example.
Check out John Culhane’s book on Fantasia from 1982. He was fortunate enough to attend the original roadshow release at the Carthay Circle Theatre in Los Angeles, CA, which employed the use of an automatically-steered Mark X unit. He confirmed somewhere in the book that when the bells began to chime towards the end of Bald Mountain, the sound did, in fact, come from the back of the room. It startled everyone.