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.
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.