Has anyone had an old soap dry out or be otherwise unsuitable for its intended purpose and if so what did you do?
I've had some Cella go rancid after a couple of years but not any tallow soaps.
Except that Cella contains tallow....
Depending on the soap, you can always try to rehydrate it. I have an old puck of Safari pressed into an Old Spice mug that's pretty dry. I always add a teaspoon or so full of water on top of the puck and let it sit for awhile, ideally overnight. The next morning I can lather it up and shave.