The notebookContentProvider API proposal is currently not on track for finalization. This api was introduced when we were still figuring out the model for working with notebooks and was inspired by the custom editor api. However we ultimately went with the much simpler NotebookSerializer API

Parts of the notebookContentProvider API are not well tested or supported today and maintaining it adds complexity to VS Code. We'd like to remove it, however the API is currently used by (or at least enabled for):

• Liveshare @alyssajotice @fubaduba (please copy correct owners if I got them wrong
• Dot net interactive @brettfo, @colombod

Please let us know if/how you are using the notebookContentProvider api and if there are any factors that would block you from migrating off of it