Meta Boxes 
Edit
Prior to the block editor, custom meta boxes were used to extend the editor. With the new editor there are new ways to extend, giving more power to the developer and a better experience for the authors. Porting older custom meta boxes to one of these new methods is encouraged as to create a more unified and consistent experience for those using the editor.
The new block editor does support most existing meta boxes, see this backward compatibility article for more support details .
Here are two mini-tutorials for creating similar functionality to meta boxes in the block editor.
Use Blocks to Store Meta Use Blocks to Store Meta
The first method is to use Blocks to store extra data with a post. The data is stored in a post meta field, similar to how meta boxes store information.
Sidebar Plugin Sidebar Plugin
If you are interested in working with the post meta outside the editor, check out the Sidebar Tutorial.