Please do comment if something is wrong or lacking, also keep it to majors unless the minor has a breaking change which mod devs who are maybe coming back to the modding scene from an older version should know about

List of versions and how they affect modders best I can tell:

1.6->1.7:

• resources became way better
  • ig block models were defined in code in 1.6
  • as well as texture atlases needing to be manually stitched instead of being a collection of 16x textures that the game stitches at runtime

1.7->1.7.2:

• string ids are now favored over numeric ids

1.7.2->1.8:

• commands
• particles are no longer handled as entities

1.8->1.9:

• balancing changes

1.9->1.10:

• idk

1.10->1.11:

• aren’t those the same version /s

1.11->1.12:

• I also don’t honestly know

1.12->1.13:

• the flattening (no more metadata, way more is possible)
• worldgen changes, start of massive rendering changes
• lots of other internal changes
• mods functionally must be completely rewritten

1.13->1.14:

• unsure, but POI came along which is useful for some mods

1.14->1.15:

• blaze3d takes over rendering in most areas of the game
• entity renderers and tile entity renderers are much easier to work with
• Any mod which does rendering needs to be slightly redone.

1.15->1.16,

• lots of worldgen changes
• post processing shaders are a part of vanilla
• refactors to how guis render (matrix stack instead of render system for translations/rotations/etc)

1.16->1.16.2:

• worldgen changes
• json biomes and dimensions(?)
• ResourceKeys

1.16->1.17:

• vertex and fragment shaders
  • these are quite useful for optimization mods as well as just in general for over the top mods
  • They also mean that the game no longer has to make a temporary variable for sake of matrix transformations, thus slightly optimizing the game’s memory footprint
• java 16
• world height limit is no longer hardcoded
• for worldgen, instances of ChunkAccess have a getMaxBuildHeight and getMinBuildHeight method, which should be used when iterating through the y axis for compatiblity with mods like CubicChunks and also to account for worldgen datapacks

(the following is provided by corgi taco)

• Feature placement method now uses a context instead of multiple arguments
  • the same is true with Carvers
• World is now in "Protochunks"
• SurfaceBuilders now have a minSurfaceHeight parameter
• Noise Step/Status of Chunk Generation now returns a CompletableFuture
  • indicates movement to multi threading of chunk generation
• WorldGenRegion is now aware of it's status
• WorldGenRegions are given a task range (I'd assume vertical height which it's meant to generate?)
• ChunkGenerator$getBaseStoneSource
  • I'd assume returns the block which surface builders should replace and the world generator should generate
• ChunkGenerator$createAquifer
  • not 100% sure about this, but I'd assume this is so that Aquifers can be generated acrossed multiple chunks without breaking due to chunk boundries