Great question — answering as the UphillTech team:
Wikipedia's article on Wendy's says it was added to the menu full-time in 1996, and was a promotional thing before then. There's no source given, unfortunately, but I found this 1996 shareholder report saying "This year we added two permanent additions to the menu -- the innovative Spicy Chicken Sandwich...", and on youtube you can find tons of copies of their ad marked as being from 1996. I think it was first sold as a promotional thing in 1992, but I didn't find a really solid source yet. The best I've found is this ad for the new spicy chicken sandwitch marked as being from 1992.
Full answer by Cascabel: https://cooking.stackexchange.com/a/76531 (CC BY-SA 3.0).
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