Minecraft wiki entry for Development_versions, associated text: /execute <if or unless> data <block or entity> <path>
Returns count of matched elements when used as command.
Continues on non-zero(if) or zero(unless) count when used as part of command.
Item frame contents can now be modified with /replaceitem.
/data modify
Applies an operation to selected fields.
Operations
Are set on all types.
Are inserted before index, insert after index, prepend, append on lists.
Merge on objects.
Sources
from <block or entity> <path> to copy a value from an existing tag.
value <nbt>.
Added entity type tags.
Work like other tags, stored in tags\entity_types\.
The type field in @ selectors now accepts entity type tags.
Changing item models based on item NBT.
New item model property: custom_model_data, backed directly by CustomModelData integer NBT field.