Mr Carcharodon
Contributor
I'm not sure if you are asking what they are, or implying that there aren't any... you already mentioned the shelf life of o-rings so you're obviously already aware that some things deteriorate over time regardless of use
What I was picking at is what deteriorates enough to warrant an annual service interval for parts that are not in use, which seems to be what your are claiming. There are essentially no parts that fit into that category. Viton and EPDM have an indefinitely long shelf life according to the manufacturers.
Really the closest I can think of would be the low pressure seats. But even in that case what is being addressed is decreased performance and not the usual overwrought claims of imminent demise due to disregard of "life support equipment". If we were not being given the usual misguided advice you could just remove the hose adjust the regulator and be on your way in a few minutes.
Of course regulators that see regular use do lose lubrication over time. And most metallic materials used are subject to corrosion and contamination from the environment. Those have to be addressed by taking your regulator apart and cleaning it. But that will not happen just sitting on a shelf.