hm, i don't really think that people get banned just for a single sentence criticizing their midlaner, but if they keep arguing or keep criticizing teammates i guess it can be categorized as harassment.
Because I've seen a few cases where some players kept criticizing their teammates on those forums, they got punished for it even if they didn't use any insults. Having a teammate contently criticizing what you're doing is annoying and when someone gets annoyed the likelihood of victory is drastically diminished.
Personally i feel like giving an advice like "Hey lux ! next time she roams bot try to ping and follow, maybe we can collapse and get her ! I'll ward near drake :p ", is way more effective than "Your job as the midlaner is to roam with the enemy midlaner or take their tower..".
To sum it up, nobody wants to listen to a complete stranger telling them what to do.
So yeah critique isn't that toxic, but it's often ineffective/useless, it can be annoying and considered as harassment if used a bunch of times or if said with CAPS LOCK on and things like that. ( This plus the fact that you'll often find critique and toxicity mixed together in the chat logs you can find on those forums ).