Mulch, menure and compost contain various microorganisms. When mulch/compost/menure is added to the garden soil, these microorganisms provide many benifits to the soil such decomposition of organic material. Many Gardners buy compost/mulch/menure in plastic bags. Sometimes we buy more than we need or buy but not use; leaving these bags for months or years somewhere in the garden, shed, or garage. I believe that if left for a long period of time, especially in a hot climate, all the microorganisms will eventually die, and thus reducing the "garden" value of these products. Of course we need this to be confirmed scientically, but if proved correct, the manufacturers of these products need to add "use by date" or "best used by date" to the packaging.
I raised this issue once with a radio gardening expert in Brisbane and my recollection is that these materials do lose their efficacy over time. Sadly I can"t recall the time frame. My assumption is that the material will still provide "bulk" to the garden that can be energised with added fertiliser. I agree that there ought to be a "best by" date on such products.