(Edit: found a link that covers the first half of what I'm talking about. It took some digging. There is no way you'd have found it with the little info you had)
https://www.nbcnews.com/storyline/isis-terror/ads-shown-isis... )
I have de-lurked because I can actually contribute to this. I am almost positive that what this is referring to is the time ISIS/ISIL (as it was still sometimes referred to then) uploaded the first video of one of their hostages (a kidnapped journalist?) being beheaded on YouTube. It would have been between 2013 and 2017 inclusive.
Advertising was in full swing on youtube with household names like Pepsi and McDonalds advertising regularly on youtube. BUT ads weren't restricted to certain types of videos then... i don't know if you were paying attention to world events then but ISIS was always in the news and when they released the beheading video it was linked EVERYWHERE. so of course when people went to go and watch a gruesome beheading, before or after it played they would see "da da da da da, I'm loving it".
There was a brief but MASSIVE public outrage against any company whose advertisements were involved, because people thought these companies were endorsing ISIS and beheadings. They didn't understand that the advertisers were paying Youtube for coverage but had no say in exactly what videos recevied what ads. They just blamed the companies they saw in connection with the video. As damage control, these major companies of course instantly pulled all ads from running on youtube and pointed the finger at YouTube, LOUDLY. Youtube lost a substantial amount of revenue and reputation pretty much overnight. Probably in less than 24 hrs.
To repair their own reputation and become an attractive and reliable investment for advertisers asap, YouTube immediately took measures to prevent this occurring again. Thus was the first purge.
I do not remember what other measures or standards were originally but they've changed over the years since. Most of the people talking about its rollon effects were youtubers talking about how it affected them personally in youtube videos, with vague or dramatic titles, which is why you would not find many results on google. They didnt want google to find them and see them criticising them and take their videos down too. I do not think the cottage industry we now have around influencers and content creation, including networking and news, had really gotten off the ground then, so nobody that i can think of would have been systematically documenting it in a written text-searchable form. Thus, no google presence.
It's really scary to me that such a major shaping event in our online lives and thus our culture has gone largely undocumented except through videos which people delist, delete, or get copyright struck down, all the time.
Tldr: Isis has a substantial share in the blame for ruining youtube. Isis is still going.