In a long-running campaign, I retired a character who had become a demigod, simply because I thought adventuring as a god was a bit silly. I had another character become an immortal (basically the same thing with different rules) who I did retire, but later went back to only because they had a lot of unfinished business I wanted to resolve. Eight years later and I still haven't managed to get them back into retirement.
I've had a character forced into retirement in an online V:tM game, something that still leaves a stale taste in my mouth- I'd been worried about their power level and had asked the staff if they were a problem, and was told they weren't repeatedly. Then I got busy IRL and had to take a month off from the game. When I came back, the entire game had been rebooted- some characters were allowed to stay with "rebalanced" (ie, nerfed) stats.
Mine was not. When I asked why, apparently my character was one of the reasons for the reboot. Like, I could see their logic, obviously, because I'd had concerns myself, but they first dismissed my concerns, then went ahead and booted me out the instant my back was turned.
: (
The only other time was when I tried to retire my Cleric in a 3e game where they had been branded a heretic and the archbishop who hated my character kept sending powerful assassins that my party ended up having to deal with, so I was ready to walk off into the sunset, but the party insisted they would stand with me.
To be fair, that was before two characters died and ended up reincarnated after a fight with a Retriever (powerful demonic construct able to plane shift, hard to kill, and can unleash nasty eye rays), but by that point the game was on it's last legs- we had one more session after that where we fought a Remorhaz, and then the DM balked at the thought of us using 5th-level spells and prestige classes so the game went on a permanent "hiatus" while he "re-evaluated" the campaign.