Placing a turntable on a load bearing wall shelf will help deal with foot fall issues and some other low frequency events but it will not isolate the turntable and might possibly make things worse depending on the design of the turntable. A wall is one big sound board. If you place the edge of a stethoscope bell against the wall you will hear everything else that is going on in the house, the dishwasher, your kid's TV up stairs and even the truck rumbling down the street. Turntables that are suspension isolated will do fine. Examples are the LP 12, SME, Basis, Avid, Thorens and Sota models. All others need to be on an isolation platform. The best is the MinusK.
Advice for mounting a TT shelf on a wall
For more than one reason, I have decided to place my TT on a wall shelf. Now it occurred to me that there may be preferable places on the wall for the shelf to be mounted. Taking into account standing waves, electrical interference (hum), and even height, I suppose that one location is preferable to another.
Also, this is a very heavy duty shelf, and there may be additional considerations just because of this. No doubt, I will locate wall studs for mounting. What considerations come to mind for this?