This number represents the . As of this writing, Minecraft 1.21 (the "Villains" or "Tricky Trials" update) changed how rendering engines handle entities. Older skin mods break in 1.21 because Mojang changed the underlying PlayerEntityModel class.
Disclaimer: This article is for educational purposes. "TL-Skin-and-Cape-Mod" is a community project. The author is not affiliated with Mojang Studios or Microsoft. File name- TL-Skin-and-Cape-Mod-Fabric-1.21.jar