If you see this message anywhere except on the F3 screen, you can ignore it. Please don't open an issue about it, the message is incorrect.

Explanation: To make detecting biome errors easier, we added an "invalid" biome named "A mod doesn't support extended biome IDs" that would show up on the F3 screen if a world-generation mod incompatible with JEID generated biomes using the old byte-based biome system. Unfortunately, many mods create "biome finder" items for each biome or otherwise list all biomes there are, resulting in this message. We'll try to rename it or remove it in a future update.