We have an Introduce Yourself forum so new people can introduce themselves and feel more with the community. In theory, a Farewell forum would do the opposite, wouldn't it. However, I honestly don't see a need for such a forum: most members bow out quietly without making too big of an affair. Plus, I don't see the need to emphasize farewells on the same level as welcomes. It makes farewells seem permanent (which, certainly isn't the case), and it's just odd in general. People coming is good, people leaving is not so good, and I don't see a need to publicize, or 'celebrate' it.
As the only member to my knowledge to post a public farewell on this forum as a topic, from personal experience, it just isn't necessary. The only reason I did so is because my absence would cause a bit of a ripple (I was a bit more popular back then and there were a lot more people), and I genuinely thought I was leaving. I came back in 5 months. People come and go from this forum time to time. Monsi pops in every once in a while, and Aeyra came back after a long hiatus. Farewells can't be permanent on the internet, where it's so easy to get back here even if you leave. And if farewells aren't permanent, we'll have a thing where "hello", "bye", "hello again" becomes prevalent.
Just my thoughts.