The way Minecraft used entity models completely changed with 1.13. In fact, a whole lot of other stuff changed behind the scenes. Optifine models for post-1.12 aren't compatible with pre-1.13 Minecraft versions and vice versa, disregarding the usual mod version-to-version compatibility. It's a similar situation with mods. Unless the mod team has the time and effort to redo all of their assets, we won't have a Pixelmon mod for versions past 1.12.2.