How about using Nextcloud with screenreaders?
Is there a accessibility auditing for the core functions and apps?
Are there accessibility guidelines for apps?
Are there tutorials for blind people?
I have seen a few bug reports on the bug tracker regarding screenreaders that were fixed (some are still open). So for end users, they try to support it as best as possible.
For the development, if they do integrated tests etc., perhaps @jan or @Daphne can tell you more. But as for all features, tests and stuff is good, but it always needs tests and feedback by users.