The bug
When the horse, donkey, mule, llama & trader llama no longer needs food, it will not make a neighs or angrily sound.
Video (Comparison of 1.15.2 and 1.16.3):
https://www.youtube.com/watch?v=fXWKbvWDoOE
Code analysis
Code analysis by @unknown can be found in this comment.
Linked issues
relates to 4
MC-183069
Donkeys, mules and undead horses cannot be saddled by right-clicking
Resolved
MC-233276
You can feed adult donkeys, horses, llamas or mules with maximum Temper value, and hand animation is not played
Resolved
MC-236341
You can feed hay bales to adult donkeys, horses or mules, but hand animation and animal eating animation/sounds are not played
Resolved
MC-279147
Attempting to use food on untamed horses, donkeys, mules, or llamas with 100 temper no longer plays a sound
Resolved
[Mod]Les3awe
Can confirm.