Disney has said that all of their properties from the Vault will be on Disney+. There’s a possibility that Song of the South could be released on there. Maybe, or maybe not. Disney put out a ride based on the film, yet no official home release.
There’s no way in hell that SotS will be on a mainstream service such as Disney+. The only, only hope that film has is via the Disney Movie Club… and even then I would be SHOCKED.