nvm, its not even playable in 1.20

I use a janky trick where each tree generates with a Chorus Flower as its supporting block instead of dirt. The Chorus Flower will immediately pop, and since I have a custom loot table that makes the Chorus Flower drop an item, I now have an entity that runs a check to make sure it's on top of dirt, under birch logs, and in a wild birch forest biome. If all of those conditions pass, it will place item frames on the tree, otherwise the item will just delete itself.

This TECHNICALLY means there's a VERY janky way to cheat and generate a bunch of tree mushrooms using a Chorus Flower, but the conditions are so difficult to artificially meet that it's really not worth it.

The data pack adds a new biome ID to the game, and that biome ID is important in saving and loading the world. By removing the data pack, the game cannot load the world, as it is looking for biome information that does not exist. Custom terrain generation is considered an experimental feature, so one should always take caution when installing any data pack that has custom biomes in their world.

I apologize for not putting any warning in the post, I'm very sorry this has caused you problems. I'll update the post now to add such a warning for future users.