Okay, I did some more research and discovered that OOTP uses the
Pitcher Abuse Points system developed by The Baseball Prospectus. This assigns pitcher abuse points once a pitcher reaches a certain number of pitches in a given game.
According to the
Injuries and Fatigue section of the OOTP 23 manual, the game considers a pitcher's past usage (Pitcher Abuse Points), pitch count in the current game, rust, and his injury proneness rating to determine the possibility of an injury.
This means that pitch counts in individual games are where you should focus, which makes perfect sense given the huge emphasis on tracking pitch counts in real-life baseball. It's also consistent with my experience in playing OOTP for many years, where I only pay attention to pitch counts in individual games. I have never paid attention to total innings pitched for the season.
Everything will vary based on the era in which you're playing. If I'm managing a team in the modern era, I pay attention to pitch counts. Otherwise, I don't. But I always pay close attention to pitcher fatigue, and I don't like keeping pitchers in the game once they're starting to tire, no matter what era it is. However, in earlier eras of baseball, pitchers in OOTP are able to throw a lot more pitches before they fatigue, so I would imagine that OOTP has scaled its Pitcher Abuse Point thresholds accordingly. But I can't say for sure how it applies that system across different eras.