ISBN now optional?

I just uploaded an update of one of my books with iTunes Producer 2.8 and got the following error message

Two things:

First of all, I had an ISBN number in my file and it was an update of the original book that is already on the iBookstore with an ISBN

Second, did something change that it is not necessary to have an ISBN number in the first place. My understanding is that the ISBN is kind of a left-over from the old-school print era and doesn't make much sense for a lot of digital publications.

Anybody with more information on that?

Thanks