Well I couldn't find a retirement announcement database online, but I had a couple of hours to kill on Amtrak, so I hacked something together. I thought my findings might be useful to someone, so I'm posting the individual cases in a spreadsheet here.
For seasons 2005-2009, I found all pitcher who threw no MLB innings, despite being projected for 60+ IP by my system. My system does not give 60 IP projections lightly (see previous post), so these cases are fairly rare. In fact, there were only 43 cases (something like 2% of all pitcher seasons), all of which I researched manually during my train ride.
Digressing for a minute, the fact that only 2% of veteran pitchers (I never project rookies to throw 60 innings) each year don't play in the majors when they are projected to pitch 60 innings is stunning. Remember, 60 IP is the average innings per MLB pitcher each year. Even with injuries, retirement, and ineffectiveness, pitchers who are projected to command average MLB innings do managed to book some MLB time 98% of the time.
Of the 43 candidates, 15 met the requirements of "voluntary retirement" that I was looking for. I was looking for pitchers who announced their retirement during the off-season or during spring training. Being cut in spring training & retiring counts. Retiring as a free agent also counts, as long as the pitcher did not appear in the minors, on the DL, or ever pitched in organized baseball again. Also two pitchers died. Lastly, I included Darrell Rasner on this list. He was sold to Japan during the offseason in a deal that he approved. He clearly could have pitched in the majors, but instead chose to go to Japan. Similarly, Kaz Ishii returned to Japan after a few years in the states. Others played in Japan after being cut in spring training by MLB teams. I did not count those guys.
I made notes on just about all of the 43 pitchers, so feel free to come to your own classifications. Also I noted whether the pitcher missed the entire (0 IP) season to injury, or whether he pitched in the minors that year. That data could be useful also.
Of course, I am not considering any voluntary retirements for pitchers projected to throw less than 60 IP. But I think that those retirements are less worthy of special consideration. It's not clear that Randy Johnson or John Smoltz could make a major league roster in 2010. Maybe they could, but it's less than clear, and I don't think anyone would project them to be anything more than a 5th starters. However Mike Mussina (and also Greg Maddux) could have absolutely commanded a starting rotation spot in 2009 if they chose to keep playing.
I'm re-training my IP models and should have new projections up soon.
As a quick look-ahead, I trained on the new data (with 15 voluntary retirements excluded from training) using the old models for IP_Start and IP_Relief as features. Relief innings projections don't change, while the new data suggests that IP_Start should be projected to be 0-2 IP higher for all pitchers. Although the later will not actually happen in my fully retrained model.
When I fully retrain, I expect to see the high-IP starters get more innings, while innings for the rest remain the same. Also, the age-based penalty for IP_Start should decrease, as voluntary retirements being excluded here mostly pertain to older pitchers