Drive through any seasonal trailer park and you will see all kinds of supports for slideouts. Then again, most of them are jacked up on concrete or cinder blocks so there is no movement at all.
My slide consists of the kitchen and the sofa. Using the sink or stove does not put any weight on the slide. Sitting on the couch does put weight on the slide. However, it is only 2' deep, so I don't see how supports under the slide would help in any way.
I saw a TV show on RVs that stated the slideout was invented in 1992. So is that different than when tipouts were used?