Thank you all very much. You input is really valuable to me and helps me a lot with my research about this topic. I am going to put everything you posted into consideration and I won't put any on opinion as the one "true" opinion. Additionaly I am very aware that some of the resources I linked in my first post sould be taken very cautiosly and your posts helped me a lot. I think my baseline will be that cheating isn't occuring a lot and I am going to take a look into the reasons why cheaters are cheating. Also thank you very much for the links and resources as I am also going to creat a timeline of cheating in speedrunning. I would just post one last question: When was the time that the community shifted from one global SDA-ruleset to a community- and game-driven ruleset for every game? I wasn't able to point this change at a specific time. And was there a global ruleset in the time before SDA started? Do you know any forums or websites from the time before SDA?
@Naxed I think this site's creation was the point where communities really started to be able to set their own rules and police themselves. Some communities were already doing this separate from SDA but speedrun.com made that model a lot more accessible and viable.
As for before SDA, I believe the big site was Twin Galaxies, which also had a global ruleset from what I remember (the website is also a pile of garbage but back then it was the only pile of garbage to work with).
I think @ShikenNuggets hit it on the head there. I joined this site a few years after it was already up and running, but from my understanding, SDA attempted to create site-wide rules based on the earliest games, namely Quake and Metroid, but as the sr community grew to encompass a wider variety of games, these early rules that worked well with some games couldn't necessarily be applied or were more difficult to apply to others. What is considered "out of bounds"? If you access an area early, is that out of bounds? Is it accessing an area the developer didn't intend? etc.
There is such a wide variety of different kinds of games out there that there is always an exception to the rule. Below is a great video explaining the history of one of these difficult to pin down definitions: what is considered "out of bounds"?
A lot of communities didn't like being pushed around and being told how to rule their own games, so they broke off and made their own site. For example, Donkey Kong Country moved here: https://dkcspeedruns.com/Main_Page.
This site (speedrun.com) was attractive because it gave rights to the runners rather than the site-wide admins under the pretense that the individual game's runners would each have more knowledge about how the rules for their respective games should be handled. SDA is still used for a lot of older games (Quake comes to mind first, but far from the only one), but I don't think you're going to find a specific time that these changed because it wasn't one big shift. Each community is in a different stage of the process and moves to wherever best fits their need.
One of the unique things about this site is that we're made up of a bunch of separate communities that aren't necessarily bound together by any rules. I want to say that this is the reason that cheating has diminished on this site; each community has its own moderator(s) to review each submission rather than a few admins (who don't necessarily even know the game) trying to spot cheating over the entire site. It guarantees that both the run will be viewed by someone who cares about the game and someone who is knowledgable about it. That's not to say cheating doesn't happen anymore, but you'd have to put so much work into cheating that you might as well learn the game and have fun playing it instead.