I still have the emails & this is what the 1st one said:
The new book cover display will start to go into effect sometime in mid-March.
You can upload your book cover at any time and it will not be a mandatory change until 7/31/2020. We encourage you to create a Book Cover and have it ready for the switch!
After 7/31/2020, Book Covers will be required in order to use Tapas similar to thumbnails. <
That sounds to me like it was available & optional in March & not mandatory until the end of April.