Correct - there is a cut musical number called "The Lorax" which was only ever incorporated into the Boston pre-Broadway tryout. I agree with the decision to cut it. Just another unnecessary subplot/story. Stick with the Horton and Jojo stories and make everything else support it. Don't use Lorax, whether MTI licenses it as optional or not.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle