I read an article by someone who had worked on transfers of Bluth films while at MGM, and he said you could see animation pegs and unpainted cel edges in the "true" full-frame image. From that it seems that NIMH and/or All Dogs Go to Heaven were shot with the entire Academy ratio exposed, but with the intent to that it would be matted to widescreen by the projectionist. Or basically, they were shot like many live action films of the same era.

(I swear I also saw frames of The Land Before Time on one of the 35mm eBay auctions that had more picture on all four sides than either the 4:3 or 16:9 versions.)