It seems likely that “appoggio” was a very different thing from what is taught today. From my point of view, I would be inclined to agree with Cornelius Reid that good singing creates its own feeling of support. That is, the singing has to develop first, and then the biofeedback of the feelings of steadiness and security will come. If there is nothing yet to support, then efforts to create support in isolation, as “preparation”, make no sense.
D. Brian Lee, from his blog article, The complicated history of F. Lamperti’s “appoggio” – Part 2