This release marks the 30th update to the editor plugin. The highlights are broken down below. Block Library
Arguably, one of the key pieces of the new editor UI is the block library or inserter. Unifying the way users can insert content has been one of the main objectives of the whole project. It has gone through multiple iterations and testing; balancing clarity, usability, flexibility, and extensibility. This release offers the result of some very fruitful collaborations between designers and developers.
Notably, tabs are being removed; blocks have bigger surface areas; and plugins can supply a distinct color for the block icon background. This seeks to allow the inserter to better scale while retaining visual clarity. The other addition to the inserter is related to the new concept of child blocks explained below.
Child Blocks
The editor has had support for nesting blocks since around the beginning of this year. With 3.0, a block author can now also register a block as being a child of another block by declaring parent: [ ‘block-name’ ]. This now causes a few changes in the interface: the root inserter won’t show child blocks unless the user is within the context of the right
Source: https://managewp.org/articles/17502/what-s-new-in-gutenberg-5th-june
Willie Chiu's Blog 