I think you're assuming that the word "feed" can only be used in a rage filled way.
The first time I heard anybody say the word "feeding" was a mid laner saying something around the lines of "Top, can we switch? I'm so far behind I'm just feeding him now."
I've said something around the lines of... "Hug the tower, otherwise you'll just feed" and "Sorry for feeding, I'm doing terrible."
Saying "stop dying" is just as helpful as "stop feeding" and just as vague. Nobody said it was intentional dying or intentional feeding (which makes the comment just plain antagonizing.) That's an issue with the person saying it and not knowing how to communicate in a helpful manner.
Calling anybody out for dying repeatedly - whether or not the word feed is used - is a negative thing to do. For example saying "Our ADC is 1-10-2." is a statement of fact. They could have also said "Our ADC is feeding." or "Our ADC keeps dying without killing anybody." All are facts. But none should be said in game because simply pointing out flaws decreases moral of a team.
The word feed in and of itself is not bad. And I don't think people assume that you're doing so intentionally. They just don't know how to communicate in an effective manner.