AlphaSkins

A long while ago Mojang removed the ability for the client to render skin's alpha channel. This broke skins that used transparency in an artistic manner.

This mod aims to reintroduce that functionality. Using mixins it turns off the code that strips the alpha channel from the skin upon rendering.

This mod doesn't make it so other people see your transparent skins. This only affects your client's rendering.

Original mixin for Fabric was found here, I just used the info found in it to port this mixin to Forge as well as added a few more to make the transparency seamless as if it was made first party.

Essentially this mod adds 3 mixins. The first one is disabling the alpha and color stripping for the skin downloading class. This class normally will set the alpha to be 0 making nothing transparent. The second mixin is a variant of the same mixin just for a SkinProcessor class, which only seems to be used for Realms-related stuff.
The third mixin patches the FirstPersonRenderer class and makes the hand respect transparency. Normally it's rendered without any care to the alpha channel but using head injection I was able to override that.

Note: I do not make any money off this link. It's fully non-monetized.