The myth is very real, mate. But at a much smaller scale than you think it is.

The amount of people who harass tanks/healers are not much. Say, in at least 10 runs I do in low-level duty roulette, I'd get all smooth runs on a good day and at max 2 runs that went bad because DPS.

The proportions is blown out of control because the tanks/healers/people that get harassed tells their side of the story and automatically blame the general community for being toxic.

The myth is real, yes. But ask the average tank/healer how many times have they actually get harassed and how many times have runs gone smooth? I bet most of them will say a lot of their runs are actually smooth and only a few them turned sour.