With the excitement of 1.17 coming out, unfortunately tragedy has struck: Mojang has patched HD heads in 1.17.
First, they updated AuthLib to completely disable the traditional method where skin files are uploaded to education.minecraft.net or bugs.mojang.com. Attempting to load skins from these areas cause this message to get thrown into the logs:
In case someone uses a way to get around the domain restriction, they have also added a cap for how big (and small) a skin texture can be, with anything that doesn't meet that standard being instantly discarded:
Code
[10:06:01] [Render thread/WARN]: Discarding incorrectly sized (2048x2048) skin texture from <link goes here>
HD heads will never be the same again. Worst. Update. Ever.