ViaBackwards/ViaVersion

Download: here

ViaBackwards/ViaVersion allows Minecraft players to join with older Minecraft versions than the server is running. Clients below 1.16 do not support custom biomes and RealisticSeasons hooks into ViaVersion to detect which players are below 1.16. Players with older versions will see the following vanilla biomes instead of the custom biome colours:

Season	Biome
Summer, Spring	Default biomes
Autumn/Fall	minecraft:savanna
Winter	minecraft:frozen_river