From r/Movie_Trivia

Apparently in the original version Bing Bong was supposed to say bitterly to Joy just before the jump ‘Don’t you understand? We are forgotten! We don’t count anymore!' but they removed these lines as it was 'too hard hitting'.

I would argue it makes the scene even more emotive to remove that line - because then Bing Bong goes to his, essentially, death with just a sense of sadness and no bitterness.. which is way worse :(

I was thinking about all the reasons the scene is upsetting - I thought maybe if I spell it out it will help to break the curse, because it still makes me tear up even to remember that scene despite having only seen the film once, 3 years ago.

Reasons it's upsetting:

Bing Bong basically dies - he will cease to exist in the memory dump.

He's a nice character who was important to Riley and doesn't deserve to die.

He already feels hurt about being mostly forgotten, sidelined and useless by his best friend.

He sacrifices himself for someone he loves.

Riley will never even know what he's done for her.

Riley won't even remember him at all, despite the massive role he played in her life as her first friend.

On a more abstract level, technically Bing Bong isn't a separate person, he's a part of Riley's pysche - so actually the message is that in order to grow up, we have to purge the innocence of our childhood.

Feel free to add any more ways this goddamn scene is so upsetting!!