I dunno. I think it depends. There are 'goodbyes' that are clearly attention seeking and the op wants people to beg and plead with her to stay and all rally round her to tear a strip off whoever has dared to offend.
Those people I think are manipulative and have no intention of going.
Then there are people who just say bye, cos they talk on here a lot and are not going to and have formed sort of friendships. Fair enough.
I've bogged off a time or two. Don't announce it because I don't expect that anyone gives a flying fuck. MN is a big place and I think unless you are in one of the little groups - the ttc or the weightloss or healthy eating etc etc, where you chat with the same people a lot, I think there's little point doing an announcement because practically nobody knows who the hell you are and those who do wouldn't notice if you were around or not!