SlotFill and Filters is a new take on the classic filters, actions, and
“SlotFill is a pattern for component extensibility, where a single Slot may be occupied by an indeterminate number of Fills elsewhere in the application,” Ryan Welcher said.
In the demo screenshot above, 10up was able to stick to the Classic UI conventions in the mobile app while displaying the same information in the Document panel of the new editor.
In January, Welcher submitted a pull request to the Gutenberg repo asking for SlotFill to be added to the document sidebar. His request received positive feedback and not only has SlotFill’s documentation been added to the WordPress Core Gutenberg Handbook, but the functionality is available in Gutenberg 6.1 and will be available in WordPress 5.3.