I left a couple forums recently but I did not post a goodbye thread, I just deleted those forums from my bookmarks and wont be visiting there any more.
Left one because the admin PM'ed me saying I wasn't interacting enough, which turned me off a little bit. I wonder if they thought a PM like that would motivate me to post more? It kind of had the opposite effect.
We have a few members here who only have time for an occasional pop-in, maybe they'll just post a quick hello in the chit-chat thread and that's it until next time they visit, which is fine by me. Do you think I am in their inbox telling them that they should be doing more than that? Nope. I appreciate the fact that they took time out of their day to say hello in the first place. Members are free to post as little or as much as they want.
The other forum I left was because the admin deleted a couple of my posts. This forum is a niche based forum (dedicated to a single subject) but they do have an off topic section, I posted a thread in their off topic section and it was deleted because it didn't fit in with the niche of the forum? Huh? Why have an off topic section if all the threads there have to be on-topic? A couple weeks before that they deleted a greeting I posted in their chit chat thread, which I let slide. I just decided to leave because I was noticing a pattern of over-moderation (not just towards me), also the forum was pretty inactive anyway.
Sometimes I'll give a forum a second chance after some time has passed which is another reason why I am not a fan of the goodbye-thread. Sometimes when I go back they're so appreciative to have another member posting, they chill out on whatever they did to turn me off in the first place. Sometimes they continue to turn me off so I'll leave again.