I'm not gonna sign some rando's petition nobody at Disney will listen to about Splash Mountain. (And I certainly won't give that rando money. What are you doing? Please let me know because maybe I should make futile internet petitions for hot topics and make some cash.) However I did want to address aspects and origins and what I think is "salvageable".
From what I can tell, the source for ZADD being based on minstrel music that was popular over fifty years before Walt was born seems be based on a book where the author asserts this is true with no real source or evidence that I can find. He then also goes on to say that Mickey Mouse's body from his shoe size to his wearing gloves is rooted in racism, though many of these same aspects were borrowed from Oswald and Felix the Cat and while Mickey didn't wear gloves until 1928, you would have to condemn a lot of characters including newer ones like Super Mario and Sonic the Hedgehog if things like big shoes and white gloves are symbols of white supremacy.
I will grant you, however, that the non-animated parts of Song of the South sound very boring and are insensitive to slavery. And the books, holy moly, the books are written (by a white man, mind you) in a sort of textualized black vernacular that makes "who dat?" sound tame; and to 2020 readers is almost painful to read. I really have no problem with the work of Joel Chandler Harris going into history's dustbin. Disney published their own copies of the Uncle Remus books with pretty artwork but, man, that text.
The characters are not solely Harris's but based in African mythology. I do still have a soft spot for Brer Rabbit, who can be found in many other symbols around the resorts (he has a tiny statue in the hub surrounding Partners.) I'd like to think with some work, they could come back in 10-20 years in a respectable fashion; but Disney would have to actually want to do that (they don't seem to like talking animals much lately) and REALLY should hire black writers to make some stories instead of adapting how 19th century whites imagined it. And Fox especially would need a lot of revisions.
The characters are public domain. Disney can do whatever they want with them, change them as they please. And as we know they own the rights to those interpretations. Unfortunately, they seem set to take the route of least resistance: pretending they were never on the wrong side of history and getting called out on it by that Warners disclaimer.