Vanilla is good - but chocolate is better. ;)
This mod exists to fix vanilla bugs - mostly to do with custom world generation - which have a much larger impact on modded.
MC-202036: Biomes IDs may become shuffled when adding or removing biomes
In vanilla, adding or removing a biome datapack may lead to shuffled biome IDs in existing worlds. This has a much larger impact on modded, as adding or removing biome mods, or even blacklisting certain biomes may cause biomes in existing worlds to shuffle, or even if a mod changed the order of their biome registrations.
Chocolate fixes this by adding a palette of biomes to each chunk when it is saved. This means chunks saved while this mod is active will never experience biome shuffling as long as the biome exists. If a biome is removed from the world, Chocolate will detect this also and re-generate the biome according to the vanilla world generation at that location (as opposed to assigning it a random biome based on the previous ID, as in vanilla).
Update 2021-01-28: Forge attempted (unsuccessfully) to fix this issue in 34.1.42. Several weeks later, they did successfully fix* this issue (as per my own testing) in 36.0.2. Their fix does not conflict with Chocolate's, which incidentally also fixes MC-197616 (which Forge does not). However, if you are here for biome ID shuffling, I recommend you just update Forge instead.
* Per my testing, there still seems to be reproduction cases of this issue, however, they will only occur with datapack-only biomes, but every reproduction case I've found thus far has been overshadowed by MC-197616 (below). Meaning that, unless you run into the below issue, you will never see biome ID shuffling.
MC-197616: Data Pack Biomes with the "Single Biome" world preset causes massive log spam and invalid client biomes.
Related to the above issue, when using the "Single Biome" world type, biomes in world do not exist in the runtime biome registry. Chocolate is able to fix biome serialization over the network in this case, which prevents the aforementioned ill effects when trying to deserialize invalid biomes on a client.
Chocolate also previously fixed MC-197860, but that has been incorporated into Forge as of 35.1.32, and as such is no longer also fixed by Chocolate.
This mod's name is a play on the "Vanilla Fix" mod as the name "Chocolate" was already taken.