We built a Google Docs writing mode some time ago, but eventually we decided not to support that writing mode anymore.
What happened is the following:
1. We deprecated our Google Docs writing mode in October 2019.
2. We had stated that we would remove the Google Docs writing mode by March 1,
3. We actually waited a lot longer than that (until January 25, 2021) to remove the Google Docs writing mode. The reason we removed it was that it was causing confusion to new authors (who thought they could use it), and it was becoming difficult to maintain.
4. We did not automatically migrate the writing mode of the handful of books which still used Google Docs writing mode, since we thought it would possibly cause more confusion.
5. We communicated this last part (#3 and #4) very poorly, due to an oversight. We will work to communicate better in future.
If you have a book that is still in the Google Docs writing mode, please see this article for our recommended next steps.
Finally, for a long explanation of our decision, please see this essay.