I guess some quick AI searches talk about the 5 man rotation becoming more common in the 1970's, but really it's more like the 80's and 90's and pretty clear to see.
re; 1969, you can just look at individual player stats, such as;
https://www.baseball-almanac.com/pla...nkife01&y=1969
https://www.baseball-almanac.com/pla...rryga01&y=1969
https://www.baseball-almanac.com/pla...laide01&y=1969
https://www.baseball-almanac.com/pla...rlejo01&y=1969
https://www.baseball-almanac.com/pla...aveto01&y=1969
https://www.baseball-almanac.com/pla...ngebi01&y=1969
https://www.baseball-almanac.com/pla...ttodo01&y=1969
https://www.baseball-almanac.com/pla...antlu01&y=1969
and on and on and on, to see all the starters going on 4 days rest and still pitching complete games, apparently not being at whatever "70% rest" status signifies and getting gassed at 75 pitches, like what was happening in the PTCS.
re; 1960's being 5 man rotations -- OK, so it's always been wrong ... so the teams weren't malformed then, there is just some inside information that isn't actually presented in the game itself for some reason or another and goes against standard rotation sizes used in the 60's.
A better question than why the rotation size is wrong, would be why this information isn't presented in the game when you're forming your team, right?
Why does it always default to 5 starters, even when asking AI to set up rotation?
... and the last two months I've been using 4 man rotations on stuff like the 1968 "Saturday Can't Sleep Clown Will Eat Me", becauase ... well, 4 man rotation was the standard in 1968, just like 1969. Why would I even think that it'd be leaving my starters at 70% effectiveness?
I guess *there* is a reason to look at the results, to find out if OOTP is using non-standard rotation sizes .. you certainly aren't provided with any other useful information