I have an ebook available through Amazon and opted to have no DRM. I'm pretty sure this is still up to the author/publisher.
I just got an E-mail about Kindle Unlimited and it had a link to make my book eligible. However, to do so you have to enroll the book in Kindle Select (or whatever it's called) and that requires that the book not be available from any other outlets.
Well, fuck that.
I can see this as preventing a good many books from being available on Kindle Unlimited.
I expect this is more of an issue for self-publishing authors with zero rep.
In any event your comment reminded me that I need to make a current backup of the books on my Kindle and strip them of any restrictions.