The block category is no longer mandatory or doesn’t have to match an existing category slug during blocks’ registration. When that happens, a block is rendered in the inserter under the Uncategorized section.
The default set of block categories was updated.
The new set of block categories is:
Text Media Design Widgets Embeds Reusable
Of the above, a few have remained unchanged: Widgets, Embeds, Reusable.
Mappings have been added to ensure that existing blocks will be assigned to as close as possible to an equivalent category in the new set:
Common → Text Formatting →