I mean, you could make an otherwise polite, respectful or reasonable comment, and the fact that it made someone leave the discussion would make it the opposite

I mean, you could make an otherwise polite, respectful or reasonable comment, and the fact that it made someone leave the discussion would make it the opposite of what it actually is, as well as "toxic". Even more, the level of "toxicity" of certain expressions would depend solely on the average tolerance of the userbase to them at any given time. You could rather label these expressions as "discussion enders", I guess, to signify the general reaction to them, rather than the nature of their content, which can vary from user to user and across time.