Is coffee considered a "stimulant laxative" (such as castor oil), or is it not considered to be a true stimulant laxative?

Coffee = irritant. In susceptible individuals, coffee can produce laxative effects within 4 minutes of drinking it. Both caffeinated and decaffeinated coffee do the same thing. It's actually an irritant, stimulating hypersecretion of gastric acids; it also speeds up gastric emptying so that acidic gastric contents get dumped into the intestine too soon. Using coffee as laxative exposes you to all this and more.
It works either way. Consuming coffee results in a rapid and well documented increase in colonic motor activity / contraction. So in that sense coffee does work as a stimulant laxative. What is unknown is the exact ingredient within coffee that causes this effect. Many people attribute it to caffeine, but studies have shown that caffeine alone cannot explain the laxative effect of coffee! the jury is still out.