Java Edition:Seed-Biome Mismatch
Every seed of a world has a specific map of biomes, which will result in the most current version. However, if the chunks were loaded in earlier versions, this map was different. This means old worlds can have biomes that are different than what they should be for that seed at that location. The biomes have changed regularly throughout the versions. All changes are listed below.
These changes are the main cause of chunk boundaries in old worlds. However, the chunk boundaries can also be caused if the biome's terrain generator is altered. Seed-biome mismatch cannot occur before 12w07a (1.2 snapshot), as this was the snapshot that introduced Anvil File Format that allowed for biomes to be saved instead of being recalculated every time the chunk is reloaded.
Overworld
Pre-Saved Changes
Before Anvil File Format's introduction in 12w07a (1.2 snapshot), every time the biomes changed, they would update in old worlds. This caused the terrain of one biome to be in the wrong biome. Before Beta 1.8 Pre-release-1, biomes were only used for generation based off rainfall and temperature. As a result, the biome did not alter the world post-generation. The modern biomes were introduced in Beta 1.8 Pre-release-1, though they still were not saved.
Pre-Anvil File Format Conversion
Biomes from before anvil file format were not saved, though some versions had a special legacy upgrading code. A world updated from 12w01a (1.1 snapshot) - 12w06a (1.2 snapshot) directly to a version 12w07a (1.2 snapshot) - 1.6.4 would have its old biomes saved. Biomes from 12w01a and 1.1 were both different than 12w03a and after because 12w03a added the jungle biome. Updating a 12w01a or 1.1 world into 12w03a - 12w06a will keep the old biomes and it to can likewise be updated to 12w07a - 1.6.4 and have its biomes retained. However, worlds created in 12w03a - 12w06a will not have their biomes saved upon updating to later versions.
Jungle Addition
Some Ice Plains Replaced with Taiga?
The Update that Changed the World
No Mutated Biomes
No Birch Forest M
New Ocean Variants
Deep Warm Ocean Removed
Bamboo Jungle Addition
Nether
Nether Uniformity
From 12w07a (and back to Alpha v1.2.0 but not saved) to 1.15.2, the Nether was a single biome, from y=0 to build limit. This biome is currently called the "Nether Wastes" (since 20w06a (1.16 snapshot)). However, from Alpha v1.2.0 to 18w16a (1.13 snapshot), it was called "Hell" and from 18w17a (1.13 snapshot) until 1.15.2, it was just called "Nether" (the actual biome ID didn't change from hell to nether until 1.13-pre5). Any chunks generated during these versions would remain fully Nether Wastes ([test] report of not updating).
Nether Biomes Introduction
From 20w06a to 20w14a (1.16 snapshots), the first pattern of the nether update would generate. 20w06a was the first snapshot in the nether update that added soul sand valleys, crimson forests, and warped forests. The addition is also the first and currently the only implementation of vertical biomes meaning that the biome can change at different y-values.
Basalt Delta Addition
From 20w15a to 20w18a (1.16 snapshots), a new pattern of biomes existed. 20w15a added basalt deltas and this biome would replace the location of others. This did not cause an overall shift of the biomes though.
[Getting different results - example: seed 8161115816409986470 x 1739 y 43 z -360 causes basalt deltas in 20w15a & 20w16a but nether wastes in 20w17a & 20w18a][test]
Less Common Basalt Deltas
From 20w19a (1.16 snapshot) to 24w38a (present),
Nether Biome Regeneration
From 20w28a to 20w29a (1.16.2 snapshots) the new nether biomes would regenerate to the map in those versions.[1] This causes the 20w06a to 20w18a (1.16 snapshots) maps to be updated to the current map.
End
End Uniformity
From 12w07a (and back to Beta 1.9 Prerelease 4 but not saved) to 18w05a (1.13 snapshot) the entire end dimension was a single biome. This biome is now called "The End" but from Beta 1.9 Prerelease 4 to 14w11b (1.8 snapshot) was called "Sky". 15w31a (1.9 snapshot) added the outer islands but their biome was still "The End". In 18w06a (1.13 snapshot) different outer island biomes were added. Currently, The End biome still generates uniformly in the center of the dimension at the main end island but in these earlier versions, the end biome could be generated uniformly at the outer islands as well.
Strict Chunk Outer Islands
The strict chunk biome layout existed from 18w06a (1.13 snapshot) to 19w35a (1.15 snapshot). In 18w06a, four new biomes were added for the outer end islands. 18w06a added End - Floating Islands (sky_island_low), The End - Medium island (sky_island_medium), The End - High island (sky_island_high), and The End - Barren island (sky_island_barren) biomes. These biomes were later renamed to Small End Islands, End Badlands, End Midlands, and End Barrens in 18w19a (1,13 snapshot). Their IDs were also changed in 1.13-pre5: sky -> end, sky_island_low -> small_end_islands, sky_island_medium -> end_midlands, sky_island_high -> end_highlands, and sky_island_barren -> end_barrens. During this version range, a bug caused these biomes to generate strictly by chunks.[2] Each chunk could only contain a single biome and the borders followed exactly along chunk borders.
Blended Chunk Outer Islands
From 19w36a (1.15 snapshot) to 24w38a (present), the biomes still generated by chunk but the boundaries were blended so that they could cross the chunk boundaries usually only one or two blocks. On boundaries, the biomes can vary in the y dimension as well, a feature first added in 19w36a.