Debatable, but the common consensus is that T-Rex had little to no feather. At the very least, the feathers couldn't have covered all of the body because T-Rex skin imprints have been found without feathers, tho they're not of all the skin, so there still may have been some feathered parts.

The idea that T-Rex had feathers didn't come from nowhere tho : We have many evidence of feathered dinosaurs from many groups. The T-Rex is niched within the coelurosauria clade, which includes many dinosaurs that are mostly covered in feathers (and even modern birds). There's even a close relative of T-Rex, Yutyrannus, with evidence of wide feather covering.

The reason why T-Rex didn't have that much feathers is likely the same reason why elephants aren't hairy : Big animals have less problem keeping heat, and may even at some point have problem evacuating excess heat (and yes, many dinosaurs were warm blooded). So as T-Rex got bigger, feathers became more of a hindrance.

[–] Jorgelino@lemmy.ml 2 points 9 months ago* (last edited 9 months ago)

I've heard that they might've been covered in feathers as children, but didn't grow any more as they got older, so they'd be spread out, not covering much, which is also how it works with elephants and hair.