Steps to reproduce:

1. Equip any slim-armed skin (e. g. Alex)
2. Compare the player models in 1.21.1 and 24w33a

Observed result:
The slim model's arms are lower than the torso by 0.5 pixels.

Expected result:
The slim model's arms are at the torso's level.

Original description:
The slim model's arms are now lower than the torso by 0.5 pixels.
This probably was a mistake because when the player holds items the arms tilt to be lined with the torso.
As on a tweet from jeb_ on 2014 showcasing this images
https://imgur.com/a/5NRZH
You can see one arm being lowered by half and the other tilted to be lined