We have reached the first WordPress core release where the post editor is no longer the only screen that uses the block editor. In the middle of the development process, we found out that several WordPress hooks defined on the server depended on the $post object that isn’t present on the updated screen that lets users edit widgets using blocks. Therefore, we decided to deprecate some of the existing filters and introduce their context-aware replacements. This way, we ensure that we can continue iteratively enabling the block-based paradigm on different screens like the navigation editor screen by leveraging the new WP_Block_Editor_Context class that will receive more capabilities over time. There are also new methods introduced that allow code reuse for the functionality that needs to be shared between the screen that uses the block editor.
Related Trac ticket: #52920.