I am a moderator in game that definitely needs a rule change, so the players must show the moment of starting a new game (previously some players were starting recording from main gameplay as it's when the timing starts). It is required because then it can be seen if the player used some option which was technically not allowed even earlier, but otherwise there is no way to tell if the player used it. So here comes my question: Is it fair to reject old runs which do not follow the new rule? Because it also wouldn't be okay to keep them but reject new runs that break it.
So you suggest keeping old runs, but at the same moment rejecting new runs because of something that old runs were allowed to do?
A common way to deal with this issue is to keep the old runs, and state in the rules something like "as of x date runs must show the new game, runs before this date are grandfathered in". That makes it clear when the rules were changed and why the old runs are allowed.
Here are a few ways to deal with the issue:
The Cat Bird community had All Worlds runs archived into a separate category once the fourth world (Zipsa) came out, since the original category only timed for three worlds. https://www.speedrun.com/cb_ce#Original_All_Worlds
In Freerice, once the website was updated, the company removed the ability Freerice 2.0, so we put all of the 2.0 runs under Misc. https://www.speedrun.com/freerice#All_Subjects_2.0
In Disney Crossy Road, the rules state "required as of 2018" to account for a few runs that were posted without audio pre-2018. A thread tracks this rule change and also updates users that timers are not required as of 2021 (but this doesn't disqualify any previous runs). https://www.speedrun.com/dcr/thread/kt9gb