When experimenting with datapacks, I found that custom crafting and smelting recipes do not appear in the recipe book until after you use them for the first time. The attached datapack, Skulls.zip, has crafting recipes for skeleton skulls, wither skulls, zombie heads, and creeper heads. They all work, however, the recipes do not appear in any recipe book until after they have been used for the first time. I've taken screenshots testing this in Survival Mode with the Skeleton and Wither skull recipes.

Be advised, the Wither skull recipe has a "cookingtime" of 20000, meaning that it takes 100 fuel points to smelt it. That's why I used a lava bucket for fuel in the test screenshots.