Minecraft 1.16.1 has introduced the ability for datapack creators to create custom dimensions and biomes. Minecraft 1.16.2 has majorly expanded on those capabilities.

As of now, Spigot doesn't use the dimensions and biomes from datapacks. However it does load custom biomes into memory.

Suggestions being self explanatory;

• Make it so that the custom dimensions befined in datapacks are being created.
• And make it so that the custom dimensions and vanilla worlds use the custom biomes defined in datapacks,

See the attached zip file for a datapack containing custom biomes.

The way minecraft handles the custom biomes can be found in net.minecraft.world.level.biome.Biome and net.minecraft.world.level.biome.BiomeManager (this is the same for the server as for the client)