My guess from what I've been seeing is the Song and Steel Dragon side bar might the closest to universally disliked as something gets in 5e, but I'm curious how much of a consensus their erasure is disliked or hated really is.
Stories tell of a wondrous time where panes of obsidian glass would show all manner of moving images; people carried small panes of manmade glass on their person that could let them speak to anyone on the planet or deliver any bit of information on any subject imaginable, and; the bodies of long-dead dragons were distilled into a serum that was burned to propel metal boxes on roads of smooth stone.
These stories are, of course, just myth and legend.
I just happen to prefer monstrous dragons rather than all dragons being hyper-intelligent immortal companions and the like and Wizards practically never wants to do those, anymore... Instead it's "Here's the same exact thing with a different color/material tied to it! Only -maybe- it's alignment is different and it blows bubbles instead of fire. Enjoy!"
For me it's like walking into a deli and finding out -every- sandwich has Mayonnaise on it. I can't stand mayo. The Turkey Bacon Ranch would be -great- just leave off the mayo just this once...? No? sigh Nevermind...
Most of that stuff is not actually new, just not updated to 5e outside of unofficial dmsguild kind of stuff. The actually new stuff is Elder Brain Dragon and Eye Drake, both of which are awesome, and maybe Dragon Blood Ooze, I don't think much else is.