- They left in the code debug features that they used to strip out.
- They left in the code their testing infrastructure that they used to strip out as well.
- They started making everything namespaced to differentiate contents between mods (like in this week's snapshot they made gamerules namespaced with the "minecraft:" prefix like items and blocks and whatnot)
- They are adding a lot more "building blocks" type features that both allow new /easier things in datapacks, and in mods as well.
Method patching with Mixins is less needed now because the game's internal APIs are more versatile than ever.