I'd say it started with Limbo tbh. I'm not here to open a can of worms debate on the frame. But the way it played and interacted with other player's kits led to toxicity of what frame you were using. Prior to Limbo NOBODY cared what frame you played ever. The community was generally receptive and welcoming. But I really think when they introduced Limbo, they inadvertently introduced a level of intolerance in the playerbase.

Many would say DE made Limbo's kit too invasive to how other players were playing. The problem with that is in the nature of the issue... by messing with other players it issue caused them severe aggravation. This led to vitriol and short novellas being written about why Limbo was the worst thing to ever exist. And of course, people who loved Limbo were going to show up to defend him This led to excessive responses riddled with toxicity and anger from both side.

It's just gotten worse from there. I honestly think that was the first major, and to this day unmended, divide in the playerbase. It seems like a trivial thing, but it's had a lasting impact over many, many years, people still quit when they see Limbo (albeit much less since many positive changes).