Don't want to write this by hand? GodCraft sets minecraft:partially_frozen for you with a switch.Get GodCraftOverview
minecraft:partially_frozen is a biome component: it goes in the components object of a custom biome. Leaves some spots unfrozen in an icy biome, giving patchy ice and snow instead of a solid sheet.
Example
Inside the components object of BP/biomes/<name>.biome.json (values are the defaults or placeholders):
BP/biomes/<name>.biome.json
{
"minecraft:partially_frozen": {}
}