From my research, it looks like the DOC came out in 2004 with the introduction of the "ACERT" engines. I'm not sure if other manufacturers (Cummins, Detroit, etc.) use them or not. In 2007, Cat switched to the (more expensive) Diesel Particulate Filter (DPF) so the DOC was only used for a short time.
I asked Dave if I could just replace it (when the time comes) with a standard muffler - I don't think the engine cares what happens to the exhaust after it leaves the engine - but Dave recommends to replace it with another DOC if/when the time comes. Hopefully I won't need to worry about that for some time.