"Tomorrow is Yesterday" for one. I do remember Spock mentioning someone with future knowledge going back in time to alter financial markets, how certain societies are run, etc.
SPOCK: “We cannot return him to Earth, Captain. He already knows too much about us and is learning more. I do not specifically refer to Captain Christopher, but suppose an unscrupulous man were to gain certain knowledge of man's future? Such a man could manipulate key industries, stocks, and even nations. and in so doing, change what must be. And if it is changed, Captain, you and I and all that we know might not even exist.” This is Spock’s personal opinion toward changing history. It is not a Starfleet rule or regulation that he is quoting.
So you don't think it's based on any rule or regulation? That it would be OK to just let people back into their time with knowledge of future events?
If it was a Starfleet rule, Spock would have quoted it. Actually, Spock wouldn’t have even needed to quote it because Kirk would already have known it. But that is not what the episode implies.
See, as a kid watching, I figured this time travel accident was the first time it had ever happened. There were no "rules" only logical speculation. This was the only Star Trek at the time. Everything else is hindsight speculation based on shows in the franchise that came AFTER.
The Temporal Prime Directive was probably at least a century away if not further into the future. In the SNW, TOS and TOS Movie Eras there was likely just a lot of guesswork and hoping that any temporal incident didn't wipe the Federation from existence. Starfleet had enough rules to allow the Enterprise to visit 1968 and conduct covert historical research but other than that I don't think many rules existed other than: "Try not to rewrite all our history books."