Kobo and Google Play both tell you up-front if a book has DRM or not. And very often - not "rare"-ly as you claim - buying directly from the publisher results in a DRM-free ebook.
All of the major eBook stores sell books with and without DRM (including Amazon Kindle, Rakuten Kobo, Apple Books, Google Play Books).
Whether an individual book includes DRM on all of these stores is down to the publisher, and you won't typically see books from the major publishers offered without DRM.
I am fairly certain that most, if not all, mass market ebooks use DRM tied to the software. If the servers go, the books will be fine for as long as the software continues to work.
There are also authors and publishers who offer books without DRM.
Please note: I do not like DRM. I simply view disinformation as a good way to loose good will from those who would otherwise support a cause.
DRM being tied to the software is effectively still the same end result. Software maker for any number of reasons stops publishing new versions of the app and they become unlisted from appstores eventually for failing to use recent SDKs or patching security flaws and users still cannot read their books.
So you depend on a company paying someone to run a DRM server or you depend on them paying someone to maintain an app and a CI/CD system. Sooner or later when the DRM content supplier stops paying for the infrastructure, your book stops working one way or the other.
Most Kindle books are not sold with DRM, for many years now. I don't know about other platforms. Readers yelled at publishers until they stopped enabling it, and turned out it didn't make enough difference to keep annoying their core customer base.
Never seen a "download DRM-free ePub" option for any Amazon book url but if they have that, the Internet seems to not know about it on a cursory search.
If you mean it downloads the book exclusively to the Kindle app in a proprietary format only that app can read, then it is just a different form of DRM a user has to hack around.
This happened with the Microsoft ebooks store and has happened with countless other DRM media.
Sometimes books are automatically revised, or censored. Sometimes accounts holding DRM licenses are deleted or deactivated by mistake.
The only way to actually own a book these days is to buy it on paper, or from one of the rare few publishers that do not use DRM.
If you want a DRM free e-book collection you will need a book scanner or send it to a scanning service.