The only message board that I left and was actively a ‘known’ member of was at Star Wars site. The board allowed politics which I actually really liked. There were lively discussions.
The problem came when the Mods for that particular board became blatant about which side of the debate they were on. Myself and other more liberal minded members of the board were routinely short term banned (usually 24 or 48 hour cooldown bans). After one banning… I just never went back.
I never think there’s any reason to publicly announce board departure. I’ve only been a regular poster on one message board in my entire life, but when I got bored of the attention whoring, pile ons, self righteousness and other miscellaneous internet shenanigans, I just stopped going. That was it. Some people felt the need to contact me asking me where I’d been or what was going on, and I’ll just assume the rest either didn’t notice or didn’t care either way.
And another thing, if not posting on an internet message board is really that big of a deal to you and you need a procedure for leaving, may I suggest going outside? I like natural sunlight.