An interesting comment on the metabox discussion WRT Gutenberg:

Does WordPress intend to formally deprecate Metabox API?
No.
[...]

Yet, there are multiple ways in which meta-boxes and extensibility could be handled:

* If we detect a meta-box is registered we can fallback to the old interface, nothing changes.
* We could split editing the content and modifying meta information into two screens or stages.
* We can try to see how feasible it is to render these as they are (PHP) below the content: #2251.
* A theme/plugin/CPT could unregister the new interface as needed.
* Various items that relied on meta-boxes could be converted to blocks for UI (still storing data separately).
* We could implement API based meta-boxes extensibility like the Fields API.

Or any combination of these.

I know a lot of people have been worried about continuing use of meta boxes in the editor view. It looks like there is no need for concern, as the team are trying to find the right way to integrate the meta boxes with the new flow that Gutenberg provides.