Version Differences

Movement: Changes or additions to movement mechanics.
Blocks: New blocks, or changes in current blocks' properties.
Other: Less important changes or additions.

This article lists all parkour-related changes in newer versions of Minecraft (1.9+).

Changes are classified in 3 categories:

1.9

Category	Changes
Movement	Jump height increased (from 1.249 to 1.252) Momentum threshold has been lowered (from 0.005 to 0.003) This is the reason why jump height is different, as well as taps. Sneaking now lowers the player's bounding box (from 1.8m to 1.65m)
Blocks	Ladders are 1 pixel wider (from 0.125b to 0.1875b)
Other	Elytras added

Category	Changes
Movement	Water movement revamped: sprinting in water now puts the player in a swimming state, which has many useful properties.
Blocks	Some blocks can now be waterlogged, allowing for more jump possibilities. Wall blocks now have a more accurate bounding box.
Other	TODO

Category	Changes
Movement	X/Z collisions changed, the first direction processed now depends on the player's velocity. Ladders and vines can now be climbed by pressing jump. Sneaking lowers the player's bounding box even more: (from 1.65m to 1.5m) You can now fit under 1.5b ceilings Forcing the player under ceilings lower than 1.5b activates the swimming state. This can be done by using a piston or trapdoor. Blips have been patched: the player can no longer abuse stepping mechanics to gain jump height. You can still clip onto the side of blocks when falling down. A new related glitch was introduced: "blip-down", which happens when activating stepping on the edge of a block. New glitch introduced: you can conserve X speed when moving into a corner.
Blocks	Movement on soulsand has been slightly tweaked. ...
Other

Category	Changes
Movement	Slipperiness changed: only blocks below 0.5b of height are affected by slippery blocks. Soulsand is no longer affected by ice and slime slipperiness.
Blocks	Honey blocks added: ...
Other	TODO